DBI(3) User Contributed Perl Documentation DBI(3)
NAME
DBI - Database independent interface for Perl
SYNOPSIS
use DBI;
@driver_names = DBI->available_drivers;
@data_sources = DBI->data_sources($driver_name, \%attr);
$dbh = DBI->connect($data_source, $username, $auth, \%attr);
$rv = $dbh->do($statement);
$rv = $dbh->do($statement, \%attr);
$rv = $dbh->do($statement, \%attr, @bind_values);
$ary_ref = $dbh->selectall_arrayref($statement);
$hash_ref = $dbh->selectall_hashref($statement, $key_field);
$ary_ref = $dbh->selectcol_arrayref($statement);
$ary_ref = $dbh->selectcol_arrayref($statement, \%attr);
$ary_ref = $dbh->selectrow_arrayref($statement);
@row_ary = $dbh->selectrow_array($statement);
$hash_ref = $dbh->selectrow_hashref($statement);
$sth = $dbh->prepare($statement);
$sth = $dbh->prepare_cached($statement);
$rv = $sth->bind_param($p_num, $bind_value);
$rv = $sth->bind_param($p_num, $bind_value, $bind_type);
$rv = $sth->bind_param($p_num, $bind_value, \%attr);
$rv = $sth->execute;
$rv = $sth->execute(@bind_values);
$rc = $sth->bind_col($col_num, \$col_variable);
$rc = $sth->bind_columns(@list_of_refs_to_vars_to_bind);
@row_ary = $sth->fetchrow_array;
$ary_ref = $sth->fetchrow_arrayref;
$hash_ref = $sth->fetchrow_hashref;
$ary_ref = $sth->fetchall_arrayref;
$ary_ref = $sth->fetchall_arrayref( { ... } );
$ary_ref = $sth->fetchall_arrayref( [ ... ] );
$hash_ref = $sth->fetchall_hashref( $key_field );
$rv = $sth->rows;
$rc = $dbh->begin_work;
$rc = $dbh->commit;
$rc = $dbh->rollback;
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$quoted_string = $dbh->quote($string);
$rc = $h->err;
$str = $h->errstr;
$rv = $h->state;
$rc = $dbh->disconnect;
This synopsis above only lists the major methods.
GETTING HELP
If you have questions about DBI, you can get help from the
dbi-users@perl.org mailing list. You can subscribe to the
list by emailing:
dbi-users-help@perl.org
Also worth a visit is the DBI home page at:
http://dbi.perl.org/
Before asking any questions, reread this document, consult
the archives and read the DBI FAQ. The archives are listed
at the end of this document and on the DBI home page. The
FAQ is installed as a the DBI::FAQ manpage module so you
can read it by executing perldoc DBI::FAQ.
Please note that Tim Bunce does not maintain the mailing
lists or the web page (generous volunteers do that). So
please don't send mail directly to him; he just doesn't
have the time to answer questions personally. The dbi-
users mailing list has lots of experienced people who
should be able to help you if you need it.
NOTE
This is the DBI specification that corresponds to the DBI
version 1.21 ($Date: 2002/02/07 03:00:53 $).
The DBI specification is evolving at a steady pace, so
it's important to check that you have the latest copy.
The significant user-visible changes in each release are
documented in the the DBI::FAQ manpage module so you can
read them by executing perldoc DBI::Changes.
Note also that whenever the DBI changes, the drivers take
some time to catch up. Recent versions of the DBI have
added new features (generally marked NEW in the text) that
may not yet be supported by the drivers you use. Talk to
the authors of those drivers if you need the new features.
Extensions to the DBI use the DBIx::* namespace. See the
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section on /Naming Conventions and Name Space and:
http://search.cpan.org/search?mode=module&query=DBIx%3A%3A
DESCRIPTION
The DBI is a database access module for the Perl
programming language. It defines a set of methods,
variables, and conventions that provide a consistent
database interface, independent of the actual database
being used.
It is important to remember that the DBI is just an
interface. The DBI is a layer of "glue" between an
application and one or more database driver modules. It
is the driver modules which do most of the real work. The
DBI provides a standard interface and framework for the
drivers to operate within.
Architecture of a DBI Application
|<- Scope of DBI ->|
.-. .--------------. .-------------.
.-------. | |---| XYZ Driver |---| XYZ Engine |
| Perl | | | `--------------' `-------------'
| script| |A| |D| .--------------. .-------------.
| using |--|P|--|B|---|Oracle Driver |---|Oracle Engine|
| DBI | |I| |I| `--------------' `-------------'
| API | | |...
|methods| | |... Other drivers
`-------' | |...
`-'
The API, or Application Programming Interface, defines the
call interface and variables for Perl scripts to use. The
API is implemented by the Perl DBI extension.
The DBI "dispatches" the method calls to the appropriate
driver for actual execution. The DBI is also responsible
for the dynamic loading of drivers, error checking and
handling, providing default implementations for methods,
and many other non-database specific duties.
Each driver contains implementations of the DBI methods
using the private interface functions of the corresponding
database engine. Only authors of sophisticated/multi-
database applications or generic library functions need be
concerned with drivers.
Notation and Conventions
The following conventions are used in this document:
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$dbh Database handle object
$sth Statement handle object
$drh Driver handle object (rarely seen or used in applications)
$h Any of the handle types above ($dbh, $sth, or $drh)
$rc General Return Code (boolean: true=ok, false=error)
$rv General Return Value (typically an integer)
@ary List of values returned from the database, typically a row of data
$rows Number of rows processed (if available, else -1)
$fh A filehandle
undef NULL values are represented by undefined values in Perl
\%attr Reference to a hash of attribute values passed to methods
Note that Perl will automatically destroy database and
statement handle objects if all references to them are
deleted.
Outline Usage
To use DBI, first you need to load the DBI module:
use DBI;
use strict;
(The use strict; isn't required but is strongly
recommended.)
Then you need to the connect entry elsewhere in this
documentto your data source and get a handle for that
connection:
$dbh = DBI->connect($dsn, $user, $password,
{ RaiseError => 1, AutoCommit => 0 });
Since connecting can be expensive, you generally just
connect at the start of your program and disconnect at the
end.
Explicitly defining the required AutoCommit behavior is
strongly recommended and may become mandatory in a later
version. This determines whether changes are
automatically committed to the database when executed, or
need to be explicitly committed later.
The DBI allows an application to "prepare" statements for
later execution. A prepared statement is identified by a
statement handle held in a Perl variable. We'll call the
Perl variable $sth in our examples.
The typical method call sequence for a SELECT statement
is:
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prepare,
execute, fetch, fetch, ...
execute, fetch, fetch, ...
execute, fetch, fetch, ...
for example:
$sth = $dbh->prepare("SELECT foo, bar FROM table WHERE baz=?");
$sth->execute( $baz );
while ( @row = $sth->fetchrow_array ) {
print "@row\n";
}
The typical method call sequence for a non-SELECT
statement is:
prepare,
execute,
execute,
execute.
for example:
$sth = $dbh->prepare("INSERT INTO table(foo,bar,baz) VALUES (?,?,?)");
while(<CSV>) {
chomp;
my ($foo,$bar,$baz) = split /,/;
$sth->execute( $foo, $bar, $baz );
}
The do() method can be used for non repeated non-SELECT
statement (or with drivers that don't support
placeholders):
$rows_affected = $dbh->do("UPDATE your_table SET foo = foo + 1");
To commit your changes to the database (when the
AutoCommit entry elsewhere in this documentis off):
$dbh->commit; # or call $dbh->rollback; to undo changes
Finally, when you have finished working with the data
source, you should the disconnect entry elsewhere in this
documentfrom it:
$dbh->disconnect;
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General Interface Rules & Caveats
The DBI does not have a concept of a "current session".
Every session has a handle object (i.e., a $dbh) returned
from the connect method. That handle object is used to
invoke database related methods.
Most data is returned to the Perl script as strings. (Null
values are returned as undef.) This allows arbitrary
precision numeric data to be handled without loss of
accuracy. Beware that Perl may not preserve the same
accuracy when the string is used as a number.
Dates and times are returned as character strings in the
current default format of the corresponding database
engine. Time zone effects are database/driver dependent.
Perl supports binary data in Perl strings, and the DBI
will pass binary data to and from the driver without
change. It is up to the driver implementors to decide how
they wish to handle such binary data.
Most databases that understand multiple character sets
have a default global charset. Text stored in the database
is, or should be, stored in that charset; if not, then
that's the fault of either the database or the application
that inserted the data. When text is fetched it should be
automatically converted to the charset of the client,
presumably based on the locale. If a driver needs to set a
flag to get that behavior, then it should do so; it should
not require the application to do that.
Multiple SQL statements may not be combined in a single
statement handle ($sth), although some databases and
drivers do support this (notably Sybase and SQL Server).
Non-sequential record reads are not supported in this
version of the DBI. In other words, records can only be
fetched in the order that the database returned them, and
once fetched they are forgotten.
Positioned updates and deletes are not directly supported
by the DBI. See the description of the CursorName
attribute for an alternative.
Individual driver implementors are free to provide any
private functions and/or handle attributes that they feel
are useful. Private driver functions can be invoked using
the DBI func() method. Private driver attributes are
accessed just like standard attributes.
Many methods have an optional \%attr parameter which can
be used to pass information to the driver implementing the
method. Except where specifically documented, the \%attr
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parameter can only be used to pass driver specific hints.
In general, you can ignore \%attr parameters or pass it as
undef.
Naming Conventions and Name Space
The DBI package and all packages below it (DBI::*) are
reserved for use by the DBI. Extensions and related
modules use the DBIx:: namespace (see
http://www.perl.com/CPAN/modules/by-module/DBIx/).
Package names beginning with DBD:: are reserved for use by
DBI database drivers. All environment variables used by
the DBI or by individual DBDs begin with "DBI_" or "DBD_".
The letter case used for attribute names is significant
and plays an important part in the portability of DBI
scripts. The case of the attribute name is used to
signify who defined the meaning of that name and its
values.
Case of name Has a meaning defined by
------------ ------------------------
UPPER_CASE Standards, e.g., X/Open, ISO SQL92 etc (portable)
MixedCase DBI API (portable), underscores are not used.
lower_case Driver or database engine specific (non-portable)
It is of the utmost importance that Driver developers only
use lowercase attribute names when defining private
attributes. Private attribute names must be prefixed with
the driver name or suitable abbreviation (e.g., "ora_" for
Oracle, "ing_" for Ingres, etc).
Driver Specific Prefix Registry:
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ad_ DBD::AnyData
ado_ DBD::ADO
best_ DBD::BestWins
csv_ DBD::CSV
db2_ DBD::DB2
f_ DBD::File
file_ DBD::TextFile
ib_ DBD::InterBase
ing_ DBD::Ingres
ix_ DBD::Informix
msql_ DBD::mSQL
mysql_ DBD::mysql
odbc_ DBD::ODBC
ora_ DBD::Oracle
pg_ DBD::Pg
proxy_ DBD::Proxy
rdb_ DBD::RDB
sapdb_ DBD::SAP_DB
solid_ DBD::Solid
syb_ DBD::Sybase
tdat_ DBD::Teradata
tuber_ DBD::Tuber
uni_ DBD::Unify
xbase_ DBD::XBase
SQL - A Query Language
Most DBI drivers require applications to use a dialect of
SQL (Structured Query Language) to interact with the
database engine. The following links provide useful
information and further links about SQL:
http://www.altavista.com/query?q=sql+tutorial
http://www.jcc.com/sql_stnd.html
http://www.contrib.andrew.cmu.edu/~shadow/sql.html
The DBI itself does not mandate or require any particular
language to be used; it is language independent. In ODBC
terms, the DBI is in "pass-thru" mode, although individual
drivers might not be. The only requirement is that queries
and other statements must be expressed as a single string
of characters passed as the first argument to the the
prepare entry elsewhere in this documentor the do entry
elsewhere in this documentmethods.
For an interesting diversion on the real history of RDBMS
and SQL, from the people who made it happen, see:
http://ftp.digital.com/pub/DEC/SRC/technical-notes/SRC-1997-018-html/sqlr95.html
Follow the "And the rest" and "Intergalactic dataspeak"
links for the SQL history.
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Placeholders and Bind Values
Some drivers support placeholders and bind values.
Placeholders, also called parameter markers, are used to
indicate values in a database statement that will be
supplied later, before the prepared statement is executed.
For example, an application might use the following to
insert a row of data into the SALES table:
INSERT INTO sales (product_code, qty, price) VALUES (?, ?, ?)
or the following, to select the description for a product:
SELECT description FROM products WHERE product_code = ?
The ? characters are the placeholders. The association of
actual values with placeholders is known as binding, and
the values are referred to as bind values.
When using placeholders with the SQL LIKE qualifier, you
must remember that the placeholder substitutes for the
whole string. So you should use "... LIKE ? ..." and
include any wildcard characters in the value that you bind
to the placeholder.
Null Values
Undefined values, or undef, can be used to indicate null
values. However, care must be taken in the particular
case of trying to use null values to qualify a SELECT
statement. Consider:
SELECT description FROM products WHERE product_code = ?
Binding an undef (NULL) to the placeholder will not select
rows which have a NULL product_code! Refer to the SQL
manual for your database engine or any SQL book for the
reasons for this. To explicitly select NULLs you have to
say "WHERE product_code IS NULL" and to make that general
you have to say:
... WHERE (product_code = ? OR (? IS NULL AND product_code IS NULL))
and bind the same value to both placeholders.
Performance
Without using placeholders, the insert statement shown
previously would have to contain the literal values to be
inserted and would have to be re-prepared and re-executed
for each row. With placeholders, the insert statement only
needs to be prepared once. The bind values for each row
can be given to the execute method each time it's called.
By avoiding the need to re-prepare the statement for each
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row, the application typically runs many times faster.
Here's an example:
my $sth = $dbh->prepare(q{
INSERT INTO sales (product_code, qty, price) VALUES (?, ?, ?)
}) or die $dbh->errstr;
while (<>) {
chomp;
my ($product_code, $qty, $price) = split /,/;
$sth->execute($product_code, $qty, $price) or die $dbh->errstr;
}
$dbh->commit or die $dbh->errstr;
See the execute and bind_param entries elsewhere in this
documentfor more details.
The q{...} style quoting used in this example avoids
clashing with quotes that may be used in the SQL
statement. Use the double-quote like qq{...} operator if
you want to interpolate variables into the string. See
the section on Quote and Quote-like Operators in the
perlop manpage for more details.
See also the the bind_column entry elsewhere in this
documentmethod, which is used to associate Perl variables
with the output columns of a SELECT statement.
THE DBI PACKAGE AND CLASS
In this section, we cover the DBI class methods, utility
functions, and the dynamic attributes associated with
generic DBI handles.
DBI Constants
Constants representing the values of the SQL standard
types can be imported individually by name, or all
together by importing the special :sql_types tag.
The names and values of all the defined SQL standard types
can be produced like this:
foreach (@{ $DBI::EXPORT_TAGS{sql_types} }) {
printf "%s=%d\n", $_, &{"DBI::$_"};
}
These constants are defined by SQL/CLI, ODBC or both.
SQL_BIGINT is (currently) omitted, because SQL/CLI and
ODBC provide conflicting codes.
See the the type_info, type_info_all, and bind_param
entries elsewhere in this documentmethods for possible
uses.
Note that just because the DBI defines a named constant
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for a given data type doesn't mean that drivers will
support that data type.
DBI Class Methods
The following methods are provided by the DBI class:
connect
$dbh = DBI->connect($data_source, $username, $password)
or die $DBI::errstr;
$dbh = DBI->connect($data_source, $username, $password, \%attr)
or die $DBI::errstr;
Establishes a database connection, or session, to the
requested $data_source. Returns a database handle
object if the connection succeeds. Use
$dbh->disconnect to terminate the connection.
If the connect fails (see below), it returns undef and
sets both $DBI::err and $DBI::errstr. (It does not set
$!, etc.) You should generally test the return status
of connect and print $DBI::errstr if it has failed.
Multiple simultaneous connections to multiple
databases through multiple drivers can be made via the
DBI. Simply make one connect call for each database
and keep a copy of each returned database handle.
The $data_source value should begin with
"dbi:driver_name:". The driver_name specifies the
driver that will be used to make the connection.
(Letter case is significant.)
As a convenience, if the $data_source parameter is
undefined or empty, the DBI will substitute the value
of the environment variable DBI_DSN. If just the
driver_name part is empty (i.e., the $data_source
prefix is "dbi::"), the environment variable
DBI_DRIVER is used. If neither variable is set, then
connect dies.
Examples of $data_source values are:
dbi:DriverName:database_name
dbi:DriverName:database_name@hostname:port
dbi:DriverName:database=database_name;host=hostname;port=port
There is no standard for the text following the driver
name. Each driver is free to use whatever syntax it
wants. The only requirement the DBI makes is that all
the information is supplied in a single string. You
must consult the documentation for the drivers you are
using for a description of the syntax they require.
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(Where a driver author needs to define a syntax for
the $data_source, it is recommended that they follow
the ODBC style, shown in the last example above.)
If the environment variable DBI_AUTOPROXY is defined
(and the driver in $data_source is not "Proxy") then
the connect request will automatically be changed to:
dbi:Proxy:$ENV{DBI_AUTOPROXY};dsn=$data_source
and passed to the DBD::Proxy module. DBI_AUTOPROXY is
typically set as "hostname=...;port=...". See the
DBD::Proxy documentation for more details.
If $username or $password are undefined (rather than
just empty), then the DBI will substitute the values
of the DBI_USER and DBI_PASS environment variables,
respectively. The DBI will warn if the environment
variables are not defined. However, the everyday use
of these environment variables is not recommended for
security reasons. The mechanism is primarily intended
to simplify testing.
DBI->connect automatically installs the driver if it
has not been installed yet. Driver installation either
returns a valid driver handle, or it dies with an
error message that includes the string
"install_driver" and the underlying problem. So
DBI->connect will die on a driver installation failure
and will only return undef on a connect failure, in
which case $DBI::errstr will hold the error message.
The $data_source argument (with the "dbi:...:" prefix
removed) and the $username and $password arguments are
then passed to the driver for processing. The DBI does
not define any interpretation for the contents of
these fields. The driver is free to interpret the
$data_source, $username, and $password fields in any
way, and supply whatever defaults are appropriate for
the engine being accessed. (Oracle, for example, uses
the ORACLE_SID and TWO_TASK environment variables if
no $data_source is specified.)
The AutoCommit and PrintError attributes for each
connection default to "on". (See the AutoCommit and
PrintError entries elsewhere in this documentfor more
information.) However, it is strongly recommended
that you explicitly define AutoCommit rather than rely
on the default. Future versions of the DBI may issue a
warning if AutoCommit is not explicitly defined.
The \%attr parameter can be used to alter the default
settings of PrintError, RaiseError, AutoCommit, and
other attributes. For example:
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$dbh = DBI->connect($data_source, $user, $pass, {
PrintError => 0,
AutoCommit => 0
});
You can also define connection attribute values within
the $data_source parameter. For example:
dbi:DriverName(PrintError=>0,Taint=>1):...
Individual attributes values specified in this way
take precedence over any conflicting values specified
via the \%attr parameter to connect.
The dbi_connect_method attribute can be used to
specify which driver method should be called to
establish the connection. The only useful values are
'connect', 'connect_cached', or some specialized case
like 'Apache::DBI::connect' (which is automatically
the default when running within Apache).
Where possible, each session ($dbh) is independent
from the transactions in other sessions. This is
useful when you need to hold cursors open across
transactions--for example, if you use one session for
your long lifespan cursors (typically read-only) and
another for your short update transactions.
For compatibility with old DBI scripts, the driver can
be specified by passing its name as the fourth
argument to connect (instead of \%attr):
$dbh = DBI->connect($data_source, $user, $pass, $driver);
In this "old-style" form of connect, the $data_source
should not start with "dbi:driver_name:". (If it does,
the embedded driver_name will be ignored). Also note
that in this older form of connect, the
$dbh->{AutoCommit} attribute is undefined, the
$dbh->{PrintError} attribute is off, and the old
DBI_DBNAME environment variable is checked if DBI_DSN
is not defined. Beware that this "old-style" connect
will be withdrawn in a future version of DBI.
connect_cached NEW
$dbh = DBI->connect_cached($data_source, $username, $password)
or die $DBI::errstr;
$dbh = DBI->connect_cached($data_source, $username, $password, \%attr)
or die $DBI::errstr;
connect_cached is like the connect entry elsewhere in
this documentexcept that the database handle returned
is also stored in a hash associated with the given
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parameters. If another call is made to connect_cached
with the same parameter values, then the corresponding
cached $dbh will be returned if it is still valid.
The cached database handle is replaced with a new
connection if it has been disconnected or if the ping
method fails.
Note that the behavior of this method differs in
several respects from the behavior of presistent
connections implemented by Apache::DBI.
Caching can be useful in some applications, but it can
also cause problems and should be used with care. The
exact behavior of this method is liable to change, so
if you intend to use it in any production applications
you should discuss your needs on the dbi-users mailing
list.
The cache can be accessed (and cleared) via the the
CachedKids entry elsewhere in this documentattribute.
available_drivers
@ary = DBI->available_drivers;
@ary = DBI->available_drivers($quiet);
Returns a list of all available drivers by searching
for DBD::* modules through the directories in @INC. By
default, a warning is given if some drivers are hidden
by others of the same name in earlier directories.
Passing a true value for $quiet will inhibit the
warning.
data_sources
@ary = DBI->data_sources($driver);
@ary = DBI->data_sources($driver, \%attr);
Returns a list of all data sources (databases)
available via the named driver. If $driver is empty
or undef, then the value of the DBI_DRIVER environment
variable is used.
The driver will be loaded if it hasn't been already.
Note that if the driver loading fails then it dies
with an error message that includes the string
"install_driver" and the underlying problem.
Data sources are returned in a form suitable for
passing to the the connect entry elsewhere in this
documentmethod (that is, they will include the
"dbi:$driver:" prefix).
Note that many drivers have no way of knowing what
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data sources might be available for it. These drivers
return an empty or incomplete list or may require
driver-specific attributes, such as a connected
database handle, to be supplied.
trace
DBI->trace($trace_level)
DBI->trace($trace_level, $trace_filename)
DBI trace information can be enabled for all handles
using the trace DBI class method. To enable trace
information for a specific handle, use the similar
$h->trace method described elsewhere.
Trace levels are as follows:
0 - Trace disabled.
1 - Trace DBI method calls returning with results or errors.
2 - Trace method entry with parameters and returning with results.
3 - As above, adding some high-level information from the driver
and some internal information from the DBI.
4 - As above, adding more detailed information from the driver.
Also includes DBI mutex information when using threaded Perl.
5 and above - As above but with more and more obscure information.
Trace level 1 is best for a simple overview of what's
happening. Trace level 2 is a good choice for general
purpose tracing. Levels 3 and above (up to 9) are
best reserved for investigating a specific problem,
when you need to see "inside" the driver and DBI.
The trace output is detailed and typically very
useful. Much of the trace output is formatted using
the the neat entry elsewhere in this documentfunction,
so strings in the trace output may be edited and
truncated.
Initially trace output is written to STDERR. If
$trace_filename is specified and can be opened in
append mode then all trace output (including that from
other handles) is redirected to that file. A warning
is generated is the file can't be opened. Further
calls to trace without a $trace_filename do not alter
where the trace output is sent. If $trace_filename is
undefined, then trace output is sent to STDERR and the
previous trace file is closed. The trace method
returns the previous tracelevel.
See also the $h->trace and $h->trace_msg methods and
the the DEBUGGING entry elsewhere in this
documentsection for information about the DBI_TRACE
environment variable.
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DBI Utility Functions
In addition to the methods listed in the previous section,
the DBI package also provides these utility functions:
neat
$str = DBI::neat($value, $maxlen);
Return a string containing a neat (and tidy)
representation of the supplied value.
Strings will be quoted, although internal quotes will
not be escaped. Values known to be numeric will be
unquoted. Undefined (NULL) values will be shown as
undef (without quotes). Unprintable characters will be
replaced by dot (.).
For result strings longer than $maxlen the result
string will be truncated to $maxlen-4 and "...'" will
be appended. If $maxlen is 0 or undef, it defaults to
$DBI::neat_maxlen which, in turn, defaults to 400.
This function is designed to format values for human
consumption. It is used internally by the DBI for the
trace entry elsewhere in this documentoutput. It
should typically not be used for formatting values for
database use. (See also the quote entry elsewhere in
this document.)
neat_list
$str = DBI::neat_list(\@listref, $maxlen, $field_sep);
Calls DBI::neat on each element of the list and
returns a string containing the results joined with
$field_sep. $field_sep defaults to ", ".
looks_like_number
@bool = DBI::looks_like_number(@array);
Returns true for each element that looks like a
number. Returns false for each element that does not
look like a number. Returns undef for each element
that is undefined or empty.
hash
$hash_value = DBI::hash($buffer, $type);
Return a 32-bit integer 'hash' value corresponding to
the contents of $buffer. The $type parameter selects
which kind of hash algorithm should be used.
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For the technically curious, type 0 (which is the
default if $type isn't specified) is based on the Perl
5.1 hash except that the value is forced to be
negative (for obscure historical reasons). Type 1 is
the better "Fowler / Noll / Vo" (FNV) hash. See
http://www.isthe.com/chongo/tech/comp/fnv/ for more
information. Both types are implemented in C and are
very fast.
This function doesn't have much to do with databases,
except that it can be handy to store hash values in a
database.
DBI Dynamic Attributes
Dynamic attributes are always associated with the last
handle used (that handle is represented by $h in the
descriptions below).
Where an attribute is equivalent to a method call, then
refer to the method call for all related documentation.
Warning: these attributes are provided as a convenience
but they do have limitations. Specifically, they have a
short lifespan: because they are associated with the last
handle used, they should only be used immediately after
calling the method that "sets" them. If in any doubt, use
the corresponding method call.
$DBI::err
Equivalent to $h->err.
$DBI::errstr
Equivalent to $h->errstr.
$DBI::state
Equivalent to $h->state.
$DBI::rows
Equivalent to $h->rows. Please refer to the
documentation for the the rows entry elsewhere in this
documentmethod.
$DBI::lasth
Returns the DBI object handle used for the most recent
DBI method call. If the last DBI method call was a
DESTROY then $DBI::lasth will return the handle of the
parent of the destroyed handle, if there is one.
METHODS COMMON TO ALL HANDLES
The following methods can be used by all types of DBI
handles.
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err
$rv = $h->err;
Returns the native database engine error code from the
last driver method called. The code is typically an
integer but you should not assume that.
The DBI resets $h->err to undef before most DBI method
calls, so the value only has a short lifespan. Also,
most drivers share the same error variables across all
their handles, so calling a method on one handle will
typically reset the error on all the other handles
that are children of that driver.
If you need to test for individual errors and have
your program be portable to different database
engines, then you'll need to determine what the
corresponding error codes are for all those engines
and test for all of them.
errstr
$str = $h->errstr;
Returns the native database engine error message from
the last driver method called. This has the same
lifespan issues as the the err entry elsewhere in this
documentmethod described above.
state
$str = $h->state;
Returns an error code in the standard SQLSTATE five
character format. Note that the specific success code
00000 is translated to '' (false). If the driver does
not support SQLSTATE (and most don't), then state will
return S1000 (General Error) for all errors.
The driver is free to return any value via state,
e.g., warning codes, even if it has not declared an
error by returning a true value via the the err entry
elsewhere in this documentmethod described above.
set_err NEW
$rv = $h->set_err($err, $errstr);
$rv = $h->set_err($err, $errstr, $state, $method);
$rv = $h->set_err($err, $errstr, $state, $method, $rv);
Set the err, errstr, and state values for the handle.
This will trigger the normal DBI error handling
mechanisms, such as RaiseError and HandleError, if
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they are enabled. This method is typically only used
by DBI drivers and DBI subclasses.
The $method parameter provides an alternate method
name, instead of the fairly unhelpful 'set_err', for
the RaiseError/PrintError error string.
The set_err method normally returns undef. The $rv
parameter provides an alternate return value. The
HandleError subroutine can access and alter this
value.
trace
$h->trace($trace_level);
$h->trace($trace_level, $trace_filename);
DBI trace information can be enabled for a specific
handle (and any future children of that handle) by
setting the trace level using the trace method.
Trace level 1 is best for a simple overview of what's
happening. Trace level 2 is a good choice for general
purpose tracing. Levels 3 and above (up to 9) are
best reserved for investigating a specific problem,
when you need to see "inside" the driver and DBI. Set
$trace_level to 0 to disable the trace.
The trace output is detailed and typically very
useful. Much of the trace output is formatted using
the the neat entry elsewhere in this documentfunction,
so strings in the trace output may be edited and
truncated.
Initially, trace output is written to STDERR. If
$trace_filename is specified, then the file is opened
in append mode and all trace output (including that
from other handles) is redirected to that file.
Further calls to trace without a $trace_filename do
not alter where the trace output is sent. If
$trace_filename is undefined, then trace output is
sent to STDERR and the previous trace file is closed.
See also the DBI->trace method, the $h->{TraceLevel}
attribute, and the DEBUGGING entry elsewhere in this
documentfor information about the DBI_TRACE
environment variable.
trace_msg
$h->trace_msg($message_text);
$h->trace_msg($message_text, $min_level);
Writes $message_text to the trace file if trace is
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enabled for $h or for the DBI as a whole. Can also be
called as DBI->trace_msg($msg). See the trace entry
elsewhere in this document.
If $min_level is defined, then the message is output
only if the trace level is equal to or greater than
that level. $min_level defaults to 1.
func
$h->func(@func_arguments, $func_name);
The func method can be used to call private non-
standard and non-portable methods implemented by the
driver. Note that the function name is given as the
last argument.
This method is not directly related to calling stored
procedures. Calling stored procedures is currently
not defined by the DBI. Some drivers, such as
DBD::Oracle, support it in non-portable ways. See
driver documentation for more details.
ATTRIBUTES COMMON TO ALL HANDLES
These attributes are common to all types of DBI handles.
Some attributes are inherited by child handles. That is,
the value of an inherited attribute in a newly created
statement handle is the same as the value in the parent
database handle. Changes to attributes in the new
statement handle do not affect the parent database handle
and changes to the database handle do not affect existing
statement handles, only future ones.
Attempting to set or get the value of an unknown attribute
is fatal, except for private driver specific attributes
(which all have names starting with a lowercase letter).
Example:
$h->{AttributeName} = ...; # set/write
... = $h->{AttributeName}; # get/read
Warn (boolean, inherited)
Enables useful warnings for certain bad practices.
Enabled by default. Some emulation layers, especially
those for Perl 4 interfaces, disable warnings. Since
warnings are generated using the Perl warn function,
they can be intercepted using the Perl $SIG{__WARN__}
hook.
Active (boolean, read-only)
True if the handle object is "active". This is rarely
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used in applications. The exact meaning of active is
somewhat vague at the moment. For a database handle it
typically means that the handle is connected to a
database ($dbh->disconnect sets Active off). For a
statement handle it typically means that the handle is
a SELECT that may have more data to fetch. (Fetching
all the data or calling $sth->finish sets Active off.)
Kids (integer, read-only)
For a driver handle, Kids is the number of currently
existing database handles that were created from that
driver handle. For a database handle, Kids is the
number of currently existing statement handles that
were created from that database handle.
ActiveKids (integer, read-only)
Like Kids, but only counting those that are Active (as
above).
CachedKids (hash ref)
For a database handle, returns a reference to the
cache (hash) of statement handles created by the the
prepare_cached entry elsewhere in this documentmethod.
For a driver handle, returns a reference to the cache
(hash) of database handles created by the the
connect_cached entry elsewhere in this documentmethod.
CompatMode (boolean, inherited)
Used by emulation layers (such as Oraperl) to enable
compatible behavior in the underlying driver (e.g.,
DBD::Oracle) for this handle. Not normally set by
application code.
InactiveDestroy (boolean)
This attribute can be used to disable the database
engine related effect of DESTROYing a handle (which
would normally close a prepared statement or
disconnect from the database etc).
For a database handle, this attribute does not disable
an explicit call to the disconnect method, only the
implicit call from DESTROY.
The default value, false, means that a handle will be
automatically destroyed when it passes out of scope.
A true value disables automatic destruction. (Think of
the name as meaning 'inactive the DESTROY method'.)
This attribute is specifically designed for use in
Unix applications that "fork" child processes. Either
the parent or the child process, but not both, should
set InactiveDestroy on all their shared handles. Note
that some databases, including Oracle, don't support
passing a database connection across a fork.
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PrintError (boolean, inherited)
This attribute can be used to force errors to generate
warnings (using warn) in addition to returning error
codes in the normal way. When set "on", any method
which results in an error occuring will cause the DBI
to effectively do a warn("$class $method failed:
$DBI::errstr") where $class is the driver class and
$method is the name of the method which failed. E.g.,
DBD::Oracle::db prepare failed: ... error text here ...
By default, DBI->connect sets PrintError "on".
If desired, the warnings can be caught and processed
using a $SIG{__WARN__} handler or modules like
CGI::Carp and CGI::ErrorWrap.
RaiseError (boolean, inherited)
This attribute can be used to force errors to raise
exceptions rather than simply return error codes in
the normal way. It is "off" by default. When set
"on", any method which results in an error will cause
the DBI to effectively do a die("$class $method
failed: $DBI::errstr"), where $class is the driver
class and $method is the name of the method that
failed. E.g.,
DBD::Oracle::db prepare failed: ... error text here ...
If you turn RaiseError on then you'd normally turn
PrintError off. If PrintError is also on, then the
PrintError is done first (naturally).
Typically RaiseError is used in conjunction with eval
{ ... } to catch the exception that's been thrown and
followed by an if ($@) { ... } block to handle the
caught exception. In that eval block the $DBI::lasth
variable can be useful for diagnosis and reporting.
For example, $DBI::lasth->{Type} and
$DBI::lasth->{Statement}.
If you want to temporarily turn RaiseError off (inside
a library function that is likely to fail, for
example), the recommended way is like this:
{
local $h->{RaiseError}; # localize and turn off for this block
...
}
The original value will automatically and reliably be
restored by Perl, regardless of how the block is
exited. The same logic applies to other attributes,
including PrintError.
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Sadly, this doesn't work for Perl versions up to and
including 5.004_04. Even more sadly, for Perl 5.5 and
5.6.0 it does work but leaks memory! For backwards
compatibility, you could just use eval { ... }
instead.
HandleError (code ref, inherited) NEW
This attribute can be used to provide your own
alternative behaviour in case of errors. If set to a
reference to a subroutine then that subroutine is
called when an error is detected (at the same point
that RaiseError and PrintError are handled).
The subroutine is called with three parameters: the
error message string that RaiseError and PrintError
would use, the DBI handle being used, and the first
value being returned by the method that failed
(typically undef).
If the subroutine returns a false value then the
RaiseError and/or PrintError attributes are checked
and acted upon as normal.
For example, to get a full stack trace for any error:
use Carp;
$h->{HandleError} = sub { confess(shift) };
Or to turn errors into exceptions:
use Exception; # or your own favourite exception module
$h->{HandleError} = sub { Exception->new('DBI')->raise($_[0]) };
It is possible to 'stack' multiple HandleError
handlers by using closures:
sub your_subroutine {
my $previous_handler = $h->{HandleError};
$h->{HandleError} = sub {
return 1 if $previous_handler and &$previous_handler(@_);
... your code here ...
}
}
Using a my inside a subroutine to store the previous
HandleError value is important. See the perlsub
manpage and the perlref manpage for more information
about closures.
It is possible for HandleError to hide an error, to a
limited degree, by using the set_err entry elsewhere
in this documentto reset $DBI::err and $DBI::errstr,
and altering the return value of the failed method.
For example:
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$h->{HandleError} = sub {
return 0 unless $_[0] =~ /^\S+ fetchrow_arrayref failed:/;
return 0 unless $_[1]->err == 1234; # the error to 'hide'
$h->set_err(0,""); # turn off the error
$_[2] = [ ... ]; # supply alternative return value
return 1;
};
This only works for methods which return a single
value and is hard to make reliable (avoiding infinite
loops, for example) and so isn't recommended for
general use! If you find a good use for it then
please let me know.
ShowErrorStatement (boolean, inherited) NEW
This attribute can be used to cause the relevant
Statement text to be appended to the error messages
generated by the RaiseError and PrintError attributes.
Only applies to errors on statement handles plus the
prepare() and do() database handle methods. (The
exact format of the appended text is subject to
change.)
TraceLevel (integer, inherited) NEW
This attribute can be used as an alternative to the
the trace entry elsewhere in this documentmethod to
set the DBI trace level for a specific handle.
FetchHashKeyName (string, inherited)
This attribute is used to specify which attribute name
the fetchrow_hashref() method should use to get the
field names for the hash keys. For historical reasons
it defaults to 'NAME' but it is recommended to set it
to 'NAME_lc' or 'NAME_uc' according to your
preference. It can only be set for driver and database
tables. For statement handles the value is frozen
when prepare() is called.
ChopBlanks (boolean, inherited)
This attribute can be used to control the trimming of
trailing space characters from fixed width character
(CHAR) fields. No other field types are affected, even
where field values have trailing spaces.
The default is false (although it is possible that the
default may change). Applications that need specific
behavior should set the attribute as needed. Emulation
interfaces should set the attribute to match the
behavior of the interface they are emulating.
Drivers are not required to support this attribute,
but any driver which does not support it must arrange
to return undef as the attribute value.
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LongReadLen (unsigned integer, inherited)
This attribute may be used to control the maximum
length of long fields ("blob", "memo", etc.) which the
driver will read from the database automatically when
it fetches each row of data. The LongReadLen
attribute only relates to fetching and reading long
values; it is not involved in inserting or updating
them.
A value of 0 means not to automatically fetch any long
data. (fetch should return undef for long fields when
LongReadLen is 0.)
The default is typically 0 (zero) bytes but may vary
between drivers. Applications fetching long fields
should set this value to slightly larger than the
longest long field value to be fetched.
Some databases return some long types encoded as pairs
of hex digits. For these types, LongReadLen relates
to the underlying data length and not the doubled-up
length of the encoded string.
Changing the value of LongReadLen for a statement
handle after it has been prepare'd will typically have
no effect, so it's common to set LongReadLen on the
$dbh before calling prepare.
Note that the value used here has a direct effect on
the memory used by the application, so don't be too
generous.
See the LongTruncOk entry elsewhere in this
documentfor more information on truncation behavior.
LongTruncOk (boolean, inherited)
This attribute may be used to control the effect of
fetching a long field value which has been truncated
(typically because it's longer than the value of the
LongReadLen attribute).
By default, LongTruncOk is false and so fetching a
long value that needs to be truncated will cause the
fetch to fail. (Applications should always be sure to
check for errors after a fetch loop in case an error,
such as a divide by zero or long field truncation,
caused the fetch to terminate prematurely.)
If a fetch fails due to a long field truncation when
LongTruncOk is false, many drivers will allow you to
continue fetching further rows.
See also the LongReadLen entry elsewhere in this
document.
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Taint (boolean, inherited)
If this attribute is set to a true value and Perl is
running in taint mode (e.g., started with the -T
option), then all data fetched from the database is
tainted, and the arguments to most DBI method calls
are checked for being tainted. This may change.
The attribute defaults to off, even if Perl is in
taint mode. See the perlsec manpage for more about
taint mode. If Perl is not running in taint mode,
this attribute has no effect.
When fetching data that you trust you can turn off the
Taint attribute, for that statement handle, for the
duration of the fetch loop.
Currently only fetched data is tainted. It is possible
that the results of other DBI method calls, and the
value of fetched attributes, may also be tainted in
future versions. That change may well break your
applications unless you take great care now. If you
use DBI Taint mode, please report your experience and
any suggestions for changes.
private_your_module_name_*
The DBI provides a way to store extra information in a
DBI handle as "private" attributes. The DBI will allow
you to store and retreive any attribute which has a
name starting with "private_".
It is strongly recommended that you use just one
private attribute (e.g., use a hash ref) and give it a
long and unambiguous name that includes the module or
application name that the attribute relates to (e.g.,
"private_YourFullModuleName_thingy").
Because of the way the Perl tie mechanism works you
cannot reliably use the ||= operator directly to
initialise the attribute, like this:
my $foo = $dbh->{private_yourmodname_foo} ||= { ... }; # WRONG
you should use a two step approach like this:
my $foo = $dbh->{private_yourmodname_foo};
$foo ||= $dbh->{private_yourmodname_foo} = { ... };
DBI DATABASE HANDLE OBJECTS
This section covers the methods and attributes associated
with database handles.
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Database Handle Methods
The following methods are specified for DBI database
handles:
do
$rows = $dbh->do($statement) or die $dbh->errstr;
$rows = $dbh->do($statement, \%attr) or die $dbh->errstr;
$rows = $dbh->do($statement, \%attr, @bind_values) or die ...
Prepare and execute a single statement. Returns the
number of rows affected or undef on error. A return
value of -1 means the number of rows is not known, not
applicable, or not available.
This method is typically most useful for non-SELECT
statements that either cannot be prepared in advance
(due to a limitation of the driver) or do not need to
be executed repeatedly. It should not be used for
SELECT statements because it does not return a
statement handle (so you can't fetch any data).
The default do method is logically similar to:
sub do {
my($dbh, $statement, $attr, @bind_values) = @_;
my $sth = $dbh->prepare($statement, $attr) or return undef;
$sth->execute(@bind_values) or return undef;
my $rows = $sth->rows;
($rows == 0) ? "0E0" : $rows; # always return true if no error
}
For example:
my $rows_deleted = $dbh->do(q{
DELETE FROM table
WHERE status = ?
}, undef, 'DONE') or die $dbh->errstr;
Using placeholders and @bind_values with the do method
can be useful because it avoids the need to correctly
quote any variables in the $statement. But if you'll
be executing the statement many times then it's more
efficient to prepare it once and call execute many
times instead.
The q{...} style quoting used in this example avoids
clashing with quotes that may be used in the SQL
statement. Use the double-quote-like qq{...} operator
if you want to interpolate variables into the string.
See the section on Quote and Quote-like Operators in
the perlop manpage for more details.
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selectrow_array
@row_ary = $dbh->selectrow_array($statement);
@row_ary = $dbh->selectrow_array($statement, \%attr);
@row_ary = $dbh->selectrow_array($statement, \%attr, @bind_values);
This utility method combines the prepare, execute,
and fetchrow_array entries elsewhere in this
documentinto a single call. If called in a list
context, it returns the first row of data from the
statement. If called in a scalar context, it returns
the first field of the first row. The $statement
parameter can be a previously prepared statement
handle, in which case the prepare is skipped.
If any method fails, and the RaiseError entry
elsewhere in this documentis not set, selectrow_array
will return an empty list.
In a scalar context, selectrow_array returns the value
of the first field. An undef is returned if there are
no matching rows or an error occurred. Since that
undef can't be distinguished from an undef returned
because the first field value was NULL, calling
selectrow_array in a scalar context should be used
with caution.
selectrow_arrayref
$ary_ref = $dbh->selectrow_array($statement);
$ary_ref = $dbh->selectrow_array($statement, \%attr);
$ary_ref = $dbh->selectrow_array($statement, \%attr, @bind_values);
This utility method combines the prepare, execute,
and fetchrow_arrayref entries elsewhere in this
documentinto a single call. It returns the first row
of data from the statement. The $statement parameter
can be a previously prepared statement handle, in
which case the prepare is skipped.
If any method fails, and the RaiseError entry
elsewhere in this documentis not set, selectrow_array
will return undef.
selectrow_hashref
$hash_ref = $dbh->selectrow_hashref($statement);
$hash_ref = $dbh->selectrow_hashref($statement, \%attr);
$hash_ref = $dbh->selectrow_hashref($statement, \%attr, @bind_values);
This utility method combines the prepare, execute,
and fetchrow_hashref entries elsewhere in this
documentinto a single call. It returns the first row
of data from the statement. The $statement parameter
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can be a previously prepared statement handle, in
which case the prepare is skipped.
If any method fails, and the RaiseError entry
elsewhere in this documentis not set,
selectrow_hashref will return undef.
selectall_arrayref
$ary_ref = $dbh->selectall_arrayref($statement);
$ary_ref = $dbh->selectall_arrayref($statement, \%attr);
$ary_ref = $dbh->selectall_arrayref($statement, \%attr, @bind_values);
This utility method combines the prepare, execute,
and fetchall_arrayref entries elsewhere in this
documentinto a single call. It returns a reference to
an array containing a reference to an array for each
row of data fetched.
The $statement parameter can be a previously prepared
statement handle, in which case the prepare is
skipped. This is recommended if the statement is going
to be executed many times.
If the RaiseError entry elsewhere in this documentis
not set and any method except fetchall_arrayref fails
then selectall_arrayref will return undef; if
fetchall_arrayref fails then it will return with
whatever data has been fetched thus far. You should
check $sth->err afterwards (or use the RaiseError
attribute) to discover if the data is complete or was
truncated due to an error.
The the fetchall_arrayref entry elsewhere in this
documentmethod called by selectall_arrayref supports a
$slice parameter. You can specify a value for $slice
by including a 'Slice' or 'Columns' attribute in
\%attr. The only difference between the two is that if
Slice is not defined and Columns is an array ref, then
the array is assumed to contain column index values
(which count from 1), rather than perl array index
values. In which case the array is copied and each
value decremented before passing to
/fetchall_arrayref.
selectall_hashref
$hash_ref = $dbh->selectall_hashref($statement, $key_field);
$hash_ref = $dbh->selectall_hashref($statement, $key_field, \%attr);
$hash_ref = $dbh->selectall_hashref($statement, $key_field, \%attr, @bind_values);
This utility method combines the prepare, execute,
and fetchall_hashref entries elsewhere in this
documentinto a single call. It returns a reference to
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a hash containing one entry for each row. The key for
each row entry is specified by $key_field. The value
is a reference to a hash returned by fetchrow_hashref.
The $statement parameter can be a previously prepared
statement handle, in which case the prepare is
skipped. This is recommended if the statement is going
to be executed many times.
If any method except fetchrow_hashref fails, and the
RaiseError entry elsewhere in this documentis not set,
selectall_hashref will return undef. If
fetchrow_hashref fails and the RaiseError entry
elsewhere in this documentis not set, then it will
return with whatever data it has fetched thus far.
$DBI::err should be checked to catch that.
selectcol_arrayref
$ary_ref = $dbh->selectcol_arrayref($statement);
$ary_ref = $dbh->selectcol_arrayref($statement, \%attr);
$ary_ref = $dbh->selectcol_arrayref($statement, \%attr, @bind_values);
This utility method combines the prepare and execute
entries elsewhere in this document and fetching one
column from all the rows, into a single call. It
returns a reference to an array containing the values
of the first column from each row.
The $statement parameter can be a previously prepared
statement handle, in which case the prepare is
skipped. This is recommended if the statement is going
to be executed many times.
If any method except fetch fails, and the RaiseError
entry elsewhere in this documentis not set,
selectcol_arrayref will return undef. If fetch fails
and the RaiseError entry elsewhere in this documentis
not set, then it will return with whatever data it has
fetched thus far. $DBI::err should be checked to catch
that.
The selectcol_arrayref method defaults to pushing a
single column value (the first) from each row into the
result array. However, it can also push another
column, or even multiple columns per row, into the
result array. This behaviour can be specified via a
'Columns' attribute which must be a ref to an array
containing the column number or numbers to use. For
example:
# get array of id and name pairs:
my $ary_ref = $dbh->selectcol_arrayref("select id, name from table", { Columns=>[1,2] });
my %hash = @$ary_ref; # build hash from key-value pairs so $hash{$id} => name
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prepare
$sth = $dbh->prepare($statement) or die $dbh->errstr;
$sth = $dbh->prepare($statement, \%attr) or die $dbh->errstr;
Prepares a single statement for later execution by the
database engine and returns a reference to a statement
handle object.
The returned statement handle can be used to get
attributes of the statement and invoke the the execute
entry elsewhere in this documentmethod. See the
section on /Statement Handle Methods.
Drivers for engines without the concept of preparing a
statement will typically just store the statement in
the returned handle and process it when $sth->execute
is called. Such drivers are unlikely to give much
useful information about the statement, such as
$sth->{NUM_OF_FIELDS}, until after $sth->execute has
been called. Portable applications should take this
into account.
In general, DBI drivers do not parse the contents of
the statement (other than simply counting any the
Placeholders entry elsewhere in this document). The
statement is passed directly to the database engine,
sometimes known as pass-thru mode. This has advantages
and disadvantages. On the plus side, you can access
all the functionality of the engine being used. On the
downside, you're limited if you're using a simple
engine, and you need to take extra care if writing
applications intended to be portable between engines.
Portable applications should not assume that a new
statement can be prepared and/or executed while still
fetching results from a previous statement.
Some command-line SQL tools use statement terminators,
like a semicolon, to indicate the end of a statement.
Such terminators should not normally be used with the
DBI.
prepare_cached
$sth = $dbh->prepare_cached($statement)
$sth = $dbh->prepare_cached($statement, \%attr)
$sth = $dbh->prepare_cached($statement, \%attr, $allow_active)
Like the prepare entry elsewhere in this
documentexcept that the statement handle returned will
be stored in a hash associated with the $dbh. If
another call is made to prepare_cached with the same
$statement and %attr values, then the corresponding
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cached $sth will be returned without contacting the
database server.
This caching can be useful in some applications, but
it can also cause problems and should be used with
care. If the cached $sth being returned is active
(i.e., is a SELECT that may still have data to be
fetched) then a warning will be generated and finish
will be called for you. The warning can be suppressed
by setting $allow_active to true. The cache can be
accessed (and cleared) via the the CachedKids entry
elsewhere in this documentattribute.
Here are some examples of prepare_cached:
sub insert_hash {
my ($table, $field_values) = @_;
my @fields = sort keys %$field_values; # sort required
my @values = @{$field_values}{@fields};
my $sql = sprintf "insert into %s (%s) values (%s)",
$table, join(",", @fields), join(",", ("?")x@fields);
my $sth = $dbh->prepare_cached($sql);
return $sth->execute(@values);
}
sub search_hash {
my ($table, $field_values) = @_;
my @fields = sort keys %$field_values; # sort required
my @values = @{$field_values}{@fields};
my $qualifier = "";
$qualifier = "where ".join(" and ", map { "$_=?" } @fields) if @fields;
$sth = $dbh->prepare_cached("SELECT * FROM $table $qualifier");
return $dbh->selectall_arrayref($sth, {}, @values);
}
commit
$rc = $dbh->commit or die $dbh->errstr;
Commit (make permanent) the most recent series of
database changes if the database supports transactions
and AutoCommit is off.
If AutoCommit is on, then calling commit will issue a
"commit ineffective with AutoCommit" warning.
See also the Transactions entry elsewhere in this
documentin the the section on /FURTHER INFORMATION
section below.
rollback
$rc = $dbh->rollback or die $dbh->errstr;
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Rollback (undo) the most recent series of uncommitted
database changes if the database supports transactions
and AutoCommit is off.
If AutoCommit is on, then calling rollback will issue
a "rollback ineffective with AutoCommit" warning.
See also the Transactions entry elsewhere in this
documentin the the section on /FURTHER INFORMATION
section below.
begin_work
$rc = $dbh->begin_work or die $dbh->errstr;
Enable transactions (by turning AutoCommit off) until
the next call to commit or rollback. After the next
commit or rollback, AutoCommit will automatically be
turned on again.
If AutoCommit is already off when begin_work is called
then it does nothing except return an error. If the
driver does not support transactions then when
begin_work attempts to set AutoCommit off the driver
will trigger a fatal error.
See also the Transactions entry elsewhere in this
documentin the the section on /FURTHER INFORMATION
section below.
disconnect
$rc = $dbh->disconnect or warn $dbh->errstr;
Disconnects the database from the database handle.
disconnect is typically only used before exiting the
program. The handle is of little use after
disconnecting.
The transaction behavior of the disconnect method is,
sadly, undefined. Some database systems (such as
Oracle and Ingres) will automatically commit any
outstanding changes, but others (such as Informix)
will rollback any outstanding changes. Applications
not using AutoCommit should explicitly call commit or
rollback before calling disconnect.
The database is automatically disconnected by the
DESTROY method if still connected when there are no
longer any references to the handle. The DESTROY
method for each driver should implicitly call rollback
to undo any uncommitted changes. This is vital
behavior to ensure that incomplete transactions don't
get committed simply because Perl calls DESTROY on
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every object before exiting. Also, do not rely on the
order of object destruction during "global
destruction", as it is undefined.
Generally, if you want your changes to be commited or
rolled back when you disconnect, then you should
explicitly call the commit entry elsewhere in this
documentor the rollback entry elsewhere in this
documentbefore disconnecting.
If you disconnect from a database while you still have
active statement handles, you will get a warning. The
statement handles should either be cleared (destroyed)
before disconnecting, or the finish method should be
called on each one.
ping
$rc = $dbh->ping;
Attempts to determine, in a reasonably efficient way,
if the database server is still running and the
connection to it is still working. Individual drivers
should implement this function in the most suitable
manner for their database engine.
The current default implementation always returns true
without actually doing anything. Actually, it returns
"0 but true" which is true but zero. That way you can
tell if the return value is genuine or just the
default. Drivers should override this method with one
that does the right thing for their type of database.
Few applications would have direct use for this
method. See the specialized Apache::DBI module for one
example usage.
get_info NEW
Warning: This method is experimental and may change.
$value = $dbh->get_info( $info_type );
Returns information about the implementation, i.e.
driver and data source capabilities, restrictions etc.
It returns undef for unknown or unimplemented
information types. For example:
$database_version = $dbh->get_info( 18 ); # SQL_DBMS_VER
$max_select_tables = $dbh->get_info( 106 ); # SQL_MAXIMUM_TABLES_IN_SELECT
See the section on /"Standards Reference Information
for more detailed information about the information
types and their meanings and possible return values.
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The DBI curently doesn't provide a name to number
mapping for the information type codes or the results.
Applications are expected to use the integer values
directly, with the name in a comment, or define their
own named values using something like the the constant
manpage pragma.
Because some DBI methods make use of get_info(),
drivers are strongly encouraged to support at least
the following very minimal set of information types to
ensure the DBI itself works properly:
Type Name Example A Example B
---- -------------------------- ------------ ------------
17 SQL_DBMS_NAME 'ACCESS' 'Oracle'
18 SQL_DBMS_VER '03.50.0000' '08.01.0721'
29 SQL_IDENTIFIER_QUOTE_CHAR '`' '"'
41 SQL_CATALOG_NAME_SEPARATOR '.' '@'
114 SQL_CATALOG_LOCATION 1 2
table_info NEW
Warning: This method is experimental and may change.
$sth = $dbh->table_info( $catalog, $schema, $table, $type );
$sth = $dbh->table_info( $catalog, $schema, $table, $type, \%attr );
$sth = $dbh->table_info( \%attr ); # old style
Returns an active statement handle that can be used to
fetch information about tables and views that exist in
the database.
The old style interface passes all the parameters as a
reference to an attribute hash with some or all of the
following attributes:
%attr = (
TABLE_CAT => $catalog # String value of the catalog name
, TABLE_SCHEM => $schema # String value of the schema name
, TABLE_NAME => $table # String value of the table name
, TABLE_TYPE => $type # String value of the table type(s)
);
The old style interface is deprecated and will be
removed in a future version.
The support for the selection criteria is driver
specific. If the driver doesn't support one or more of
them then you may get back more than you asked for and
can do the filtering yourself.
The arguments $catalog, $schema and $table may accept
search patterns according to the database/driver, for
example: $table = '%FOO%'; Remember that the
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underscore character ('_') is a search pattern that
means match any character, so 'FOO_%' is the same as
'FOO%' and 'FOO_BAR%' will match names like 'FOO1BAR'.
The value of $type is a comma-separated list of one or
more types of tables to be returned in the result set.
Each value may optionally be quoted, e.g.:
$type = "TABLE";
$type = "'TABLE','VIEW'";
In addition the following special cases may also be
supported by some drivers:
o If the value of $catalog is '%' and $schema and $table
name are empty strings, the result set contains a
list of catalog names. For example:
$sth = $dbh->table_info('%', '', '');
o If the value of $schema is '%' and $catalog and $table
are empty strings, the result set contains a list
of schema names.
o If the value of $type is '%' and $catalog, $schema, and
$table are all empty strings, the result set
contains a list of table types.
The statement handle returned has at least the
following fields in the order show below. Other
fields, after these, may also be present.
TABLE_CAT: Table catalog identifier. This field is
NULL (undef) if not applicable to the data source,
which is usually the case. This field is empty if
not applicable to the table.
TABLE_SCHEM: The name of the schema containing the
TABLE_NAME value. This field is NULL (undef) if
not applicable to data source, and empty if not
applicable to the table.
TABLE_NAME: Name of the table (or view, synonym,
etc).
TABLE_TYPE: One of the following: "TABLE", "VIEW",
"SYSTEM TABLE", "GLOBAL TEMPORARY", "LOCAL
TEMPORARY", "ALIAS", "SYNONYM" or a type
identifier that is specific to the data source.
REMARKS: A description of the table. May be NULL
(undef).
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Note that table_info might not return records for
all tables. Applications can use any valid table
regardless of whether it's returned by table_info.
See also the tables entry elsewhere in this
document and the section on /"Standards Reference
Information.
column_info NEW
Warning: This method is experimental and may change.
$sth = $dbh->column_info( $catalog, $schema, $table, $column );
Returns an active statement handle that can be used to
fetch information about columns in specified tables.
The arguments $schema, $table and $column may accept
search patterns according to the database/driver, for
example: $table = '%FOO%';
Note: The support for the selection criteria is driver
specific. If the driver doesn't support one or more of
them then you may get back more than you asked for and
can do the filtering yourself.
The statement handle returned has at least the
following fields in the order shown below. Other
fields, after these, may also be present.
TABLE_CAT: The catalog identifier. This field is NULL
(undef) if not applicable to the data source, which is
often the case. This field is empty if not applicable
to the table.
TABLE_SCHEM: The schema identifier. This field is
NULL (undef) if not applicable to the data source, and
empty if not applicable to the table.
TABLE_NAME: The table identifier. Note: A driver may
provide column metadata not only for base tables, but
also for derived objects like SYNONYMS etc.
COLUMN_NAME: The column identifier.
DATA_TYPE: The concise data type code.
TYPE_NAME: A data source dependent data type name.
COLUMN_SIZE: The column size. This is the maximum
length in characters for character data types, the
number of digits or bits for numeric data types or the
length in the representation of temporal types. See
the relevant specifications for detailed information.
BUFFER_LENGTH: The length in bytes of transferred
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data.
DECIMAL_DIGITS: The total number of significant digits
to the right of the decimal point.
NUM_PREC_RADIX: The radix for numeric precision. The
value is 10 or 2 for numeric data types and NULL
(undef) if not applicable.
NULLABLE: Indicates if a column can accept NULLs. The
following values are defined:
SQL_NO_NULLS 0
SQL_NULLABLE 1
SQL_NULLABLE_UNKNOWN 2
REMARKS: A description of the column.
COLUMN_DEF: The default value of the column.
SQL_DATA_TYPE: The SQL data type.
SQL_DATETIME_SUB: The subtype code for datetime and
interval data types.
CHAR_OCTET_LENGTH: The maximum length in bytes of a
character or binary data type column.
ORDINAL_POSITION: The column sequence number (starting
with 1).
IS_NULLABLE: Indicates if the column can accept NULLs.
Possible values are: 'NO', 'YES' and ''.
SQL/CLI defines the following additional columns:
CHAR_SET_CAT
CHAR_SET_SCHEM
CHAR_SET_NAME
COLLATION_CAT
COLLATION_SCHEM
COLLATION_NAME
UDT_CAT
UDT_SCHEM
UDT_NAME
DOMAIN_CAT
DOMAIN_SCHEM
DOMAIN_NAME
SCOPE_CAT
SCOPE_SCHEM
SCOPE_NAME
MAX_CARDINALITY
DTD_IDENTIFIER
IS_SELF_REF
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Drivers capable of supplying any of those values
should do so in the corresponding column and supply
undef values for the others.
Drivers wishing to provide extra database/driver
specific information should do so in extra columns
beyond all those listed above, and use lowercase field
names with the driver-specific prefix (i.e.,
'ora_...'). Applications accessing such fields should
do so by name and not by column number.
The result set is ordered by TABLE_CAT, TABLE_SCHEM,
TABLE_NAME and ORDINAL_POSITION.
Note: There is some overlap with statement attributes
(in perl) and SQLDescribeCol (in ODBC). However,
SQLColumns provides more metadata.
See also the section on /"Standards Reference
Information.
primary_key_info NEW
Warning: This method is experimental and may change.
$sth = $dbh->primary_key_info( $catalog, $schema, $table );
Returns an active statement handle that can be used to
fetch information about columns that make up the
primary key for a table. The arguments don't accept
search patterns (unlike table_info()).
For example:
$sth = $dbh->primary_key_info( undef, $user, 'foo' );
$data = $sth->fetchall_arrayref;
Note: The support for the selection criteria, such as
$catalog, is driver specific. If the driver doesn't
support catalogs and/or schemas, it may ignore these
criteria.
The statement handle returned has at least the
following fields in the order shown below. Other
fields, after these, may also be present.
TABLE_CAT: The catalog identifier. This field is NULL
(undef) if not applicable to the data source, which is
often the case. This field is empty if not applicable
to the table.
TABLE_SCHEM: The schema identifier. This field is
NULL (undef) if not applicable to the data source, and
empty if not applicable to the table.
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TABLE_NAME: The table identifier.
COLUMN_NAME: The column identifier.
KEY_SEQ: The column sequence number (starting with 1).
Note: This field is named ORDINAL_POSITION in SQL/CLI.
PK_NAME: The primary key constraint identifier. This
field is NULL (undef) if not applicable to the data
source.
See also the section on /"Standards Reference
Information.
primary_key NEW
Warning: This method is experimental and may change.
@key_column_names = $dbh->primary_key( $catalog, $schema, $table );
Simple interface to the primary_key_info() method.
Returns a list of the column names that comprise the
primary key of the specified table. The list is in
primary key column sequence order.
foreign_key_info NEW
Warning: This method is experimental and may change.
$sth = $dbh->foreign_key_info( $pk_catalog, $pk_schema, $pk_table
, $fk_catalog, $fk_schema, $fk_table );
Returns an active statement handle that can be used to
fetch information about foreign keys in and/or
referencing the specified table(s). The arguments
don't accept search patterns (unlike table_info()).
$pk_catalog, $pk_schema, $pk_table identify the
primary (unique) key table (PKT).
$fk_catalog, $fk_schema, $fk_table identify the
foreign key table (FKT).
If both PKT and FKT are given, the function returns
the foreign key, if any, in table FKT that refers to
the primary (unique) key of table PKT. (Note: In
SQL/CLI, the result is implementation-defined.)
If only PKT is given, then the result set contains the
primary key of that table and all foreign keys that
refer to it.
If only FKT is given, then the result set contains all
foreign keys in that table and the primary keys to
which they refer. (Note: In SQL/CLI, the result
includes unique keys too.)
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For example:
$sth = $dbh->foreign_key_info( undef, $user, 'master');
$sth = $dbh->foreign_key_info( undef, undef, undef , undef, $user, 'detail');
$sth = $dbh->foreign_key_info( undef, $user, 'master', undef, $user, 'detail');
Note: The support for the selection criteria, such as
$catalog, is driver specific. If the driver doesn't
support catalogs and/or schemas, it may ignore these
criteria.
The statement handle returned has the following fields
in the order shown below. Because ODBC never includes
unique keys, they define different columns in the
result set than SQL/CLI. SQL/CLI column names are
shown in parentheses.
PKTABLE_CAT ( UK_TABLE_CAT ): The primary
(unique) key table catalog identifier. This field is
NULL (undef) if not applicable to the data source,
which is often the case. This field is empty if not
applicable to the table.
PKTABLE_SCHEM ( UK_TABLE_SCHEM ): The primary
(unique) key table schema identifier. This field is
NULL (undef) if not applicable to the data source, and
empty if not applicable to the table.
PKTABLE_NAME ( UK_TABLE_NAME ): The primary
(unique) key table identifier.
PKCOLUMN_NAME (UK_COLUMN_NAME ): The primary
(unique) key column identifier.
FKTABLE_CAT ( FK_TABLE_CAT ): The foreign key
table catalog identifier. This field is NULL (undef)
if not applicable to the data source, which is often
the case. This field is empty if not applicable to
the table.
FKTABLE_SCHEM ( FK_TABLE_SCHEM ): The foreign key
table schema identifier. This field is NULL (undef)
if not applicable to the data source, and empty if not
applicable to the table.
FKTABLE_NAME ( FK_TABLE_NAME ): The foreign key
table identifier.
FKCOLUMN_NAME ( FK_COLUMN_NAME ): The foreign key
column identifier.
KEY_SEQ ( ORDINAL_POSITION ): The column
sequence number (starting with 1).
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UPDATE_RULE ( UPDATE_RULE ): The referential
action for the UPDATE rule. The following codes are
defined:
CASCADE 0
RESTRICT 1
SET NULL 2
NO ACTION 3
SET DEFAULT 4
DELETE_RULE ( DELETE_RULE ): The referential
action for the DELETE rule. The codes are the same as
for UPDATE_RULE.
FK_NAME ( FK_NAME ): The foreign key
name.
PK_NAME ( UK_NAME ): The primary
(unique) key name.
DEFERRABILITY ( DEFERABILITY ): The
deferrability of the foreign key constraint. The
following codes are defined:
INITIALLY DEFERRED 5
INITIALLY IMMEDIATE 6
NOT DEFERRABLE 7
( UNIQUE_OR_PRIMARY ): This column is
necessary if a driver includes all candidate (i.e.
primary and alternate) keys in the result set (as
specified by SQL/CLI). The value of this column is
UNIQUE if the foreign key references an alternate key
and PRIMARY if the foreign key references a primary
key, or it may be undefined if the driver doesn't have
access to the information.
See also the section on /"Standards Reference
Information.
tables NEW
Warning: This method is experimental and may change.
@names = $dbh->tables( $catalog, $schema, $table, $type );
@names = $dbh->tables; # deprecated
Simple interface to table_info(). Returns a list of
matching table names, possibly including a
catalog/schema prefix.
See the table_info entry elsewhere in this documentfor
a description of the parameters.
If $dbh->get_info(29) returns true (29 is
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SQL_IDENTIFIER_QUOTE_CHAR) then the table names are
constructed and quoted by the quote_identifier entry
elsewhere in this documentto ensure they are usable
even if they contain whitespace or reserved words etc.
type_info_all
Warning: This method is experimental and may change.
$type_info_all = $dbh->type_info_all;
Returns a reference to an array which holds
information about each data type variant supported by
the database and driver. The array and its contents
should be treated as read-only.
The first item is a reference to an 'index' hash of
Name => Index pairs. The items following that are
references to arrays, one per supported data type
variant. The leading index hash defines the names and
order of the fields within the arrays that follow it.
For example:
$type_info_all = [
{ TYPE_NAME => 0,
DATA_TYPE => 1,
COLUMN_SIZE => 2, # was PRECISION originally
LITERAL_PREFIX => 3,
LITERAL_SUFFIX => 4,
CREATE_PARAMS => 5,
NULLABLE => 6,
CASE_SENSITIVE => 7,
SEARCHABLE => 8,
UNSIGNED_ATTRIBUTE=> 9,
FIXED_PREC_SCALE => 10, # was MONEY originally
AUTO_UNIQUE_VALUE => 11, # was AUTO_INCREMENT originally
LOCAL_TYPE_NAME => 12,
MINIMUM_SCALE => 13,
MAXIMUM_SCALE => 14,
NUM_PREC_RADIX => 15,
},
[ 'VARCHAR', SQL_VARCHAR,
undef, "'","'", undef,0, 1,1,0,0,0,undef,1,255, undef
],
[ 'INTEGER', SQL_INTEGER,
undef, "", "", undef,0, 0,1,0,0,0,undef,0, 0, 10
],
];
Note that more than one row may have the same value in
the DATA_TYPE field if there are different ways to
spell the type name and/or there are variants of the
type with different attributes (e.g., with and without
AUTO_UNIQUE_VALUE set, with and without
UNSIGNED_ATTRIBUTE, etc).
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The rows are ordered by DATA_TYPE first and then by
how closely each type maps to the corresponding ODBC
SQL data type, closest first.
The meaning of the fields is described in the
documentation for the the type_info entry elsewhere in
this documentmethod. The index values shown above
(e.g., NULLABLE => 6) are for illustration only.
Drivers may define the fields with a different order.
This method is not normally used directly. The the
type_info entry elsewhere in this documentmethod
provides a more useful interface to the data.
Even though an 'index' hash is provided, all the field
names in the index hash defined above will always have
the index values defined above. This is defined
behaviour so that you don't need to rely on the index
hash, which is handy because the lettercase of the
keys is not defined. It is usually uppercase, as show
here, but drivers are free to return names with any
lettercase. Drivers are also free to return extra
driver-specific columns of information - though it's
recommended that they start at column index 50 to
leave room for expansion of the DBI/ODBC
specification.
type_info
Warning: This method is experimental and may change.
@type_info = $dbh->type_info($data_type);
Returns a list of hash references holding information
about one or more variants of $data_type. The list is
ordered by DATA_TYPE first and then by how closely
each type maps to the corresponding ODBC SQL data
type, closest first. If called in a scalar context
then only the first (best) element is returned.
If $data_type is undefined or SQL_ALL_TYPES, then the
list will contain hashes for all data type variants
supported by the database and driver.
If $data_type is an array reference then type_info
returns the information for the first type in the
array that has any matches.
The keys of the hash follow the same letter case
conventions as the rest of the DBI (see the section on
/Naming Conventions and Name Space). The following
items should exist:
TYPE_NAME (string)
Data type name for use in CREATE TABLE statements
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etc.
DATA_TYPE (integer)
SQL data type number.
COLUMN_SIZE (integer)
For numeric types, this is either the total number
of digits (if the NUM_PREC_RADIX value is 10) or
the total number of bits allowed in the column (if
NUM_PREC_RADIX is 2).
For string types, this is the maximum size of the
string in bytes.
For date and interval types, this is the maximum
number of characters needed to display the value.
LITERAL_PREFIX (string)
Characters used to prefix a literal. A typical
prefix is "'" for characters, or possibly "0x" for
binary values passed as hexadecimal. NULL (undef)
is returned for data types for which this is not
applicable.
LITERAL_SUFFIX (string)
Characters used to suffix a literal. Typically "'"
for characters. NULL (undef) is returned for data
types where this is not applicable.
CREATE_PARAMS (string)
Parameter names for data type definition. For
example, CREATE_PARAMS for a DECIMAL would be
"precision,scale" if the DECIMAL type should be
declared as DECIMAL(precision,scale) where
precision and scale are integer values. For a
VARCHAR it would be "max length". NULL (undef) is
returned for data types for which this is not
applicable.
NULLABLE (integer)
Indicates whether the data type accepts a NULL
value: 0 or an empty string = no, 1 = yes, 2 =
unknown.
CASE_SENSITIVE (boolean)
Indicates whether the data type is case sensitive
in collations and comparisons.
SEARCHABLE (integer)
Indicates how the data type can be used in a WHERE
clause, as follows:
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0 - Cannot be used in a WHERE clause
1 - Only with a LIKE predicate
2 - All comparison operators except LIKE
3 - Can be used in a WHERE clause with any comparison operator
UNSIGNED_ATTRIBUTE (boolean)
Indicates whether the data type is unsigned. NULL
(undef) is returned for data types for which this
is not applicable.
FIXED_PREC_SCALE (boolean)
Indicates whether the data type always has the
same precision and scale (such as a money type).
NULL (undef) is returned for data types for which
this is not applicable.
AUTO_UNIQUE_VALUE (boolean)
Indicates whether a column of this data type is
automatically set to a unique value whenever a new
row is inserted. NULL (undef) is returned for
data types for which this is not applicable.
LOCAL_TYPE_NAME (string)
Localized version of the TYPE_NAME for use in
dialog with users. NULL (undef) is returned if a
localized name is not available (in which case
TYPE_NAME should be used).
MINIMUM_SCALE (integer)
The minimum scale of the data type. If a data type
has a fixed scale, then MAXIMUM_SCALE holds the
same value. NULL (undef) is returned for data
types for which this is not applicable.
MAXIMUM_SCALE (integer)
The maximum scale of the data type. If a data type
has a fixed scale, then MINIMUM_SCALE holds the
same value. NULL (undef) is returned for data
types for which this is not applicable.
SQL_DATA_TYPE (integer)
This column is the same as the DATA_TYPE column,
except for interval and datetime data types. For
interval and datetime data types, the
SQL_DATA_TYPE field will return SQL_INTERVAL or
SQL_DATETIME, and the SQL_DATETIME_SUB field below
will return the subcode for the specific interval
or datetime data type. If this field is NULL, then
the driver does not support or report on interval
or date subtypes.
SQL_DATETIME_SUB (integer)
For interval or datetime data types, where the
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SQL_DATA_TYPE field above is SQL_INTERVAL or
SQL_DATETIME, this field will hold the subcode for
the specific interval or datetime data type.
Otherwise it will be NULL (undef).
NUM_PREC_RADIX (integer)
The radix value of the data type. For approximate
numeric types, NUM_PREC_RADIX contains the value 2
and COLUMN_SIZE holds the number of bits. For
exact numeric types, NUM_PREC_RADIX contains the
value 10 and COLUMN_SIZE holds the number of
decimal digits. NULL (undef) is returned either
for data types for which this is not applicable or
if the driver cannot report this information.
INTERVAL_PRECISION (integer)
The interval leading precision for interval types.
NULL is returned either for data types for which
this is not applicable or if the driver cannot
report this information.
For example, to find the type name for the fields
in a select statement you can do:
@names = map { scalar $dbh->type_info($_)->{TYPE_NAME} } @{ $sth->{TYPE} }
Since DBI and ODBC drivers vary in how they map
their types into the ISO standard types you may
need to search for more than one type. Here's an
example looking for a usable type to store a date:
$my_date_type = $dbh->type_info( [ SQL_DATE, SQL_TIMESTAMP ] );
Similarly, to more reliably find a type to store
small integers, you could use a list starting with
SQL_SMALLINT, SQL_INTEGER, SQL_DECIMAL, etc.
See also the section on /"Standards Reference
Information.
quote
$sql = $dbh->quote($value);
$sql = $dbh->quote($value, $data_type);
Quote a string literal for use as a literal value in
an SQL statement, by escaping any special characters
(such as quotation marks) contained within the string
and adding the required type of outer quotation marks.
$sql = sprintf "SELECT foo FROM bar WHERE baz = %s",
$dbh->quote("Don't");
For most database types, quote would return 'Don''t'
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(including the outer quotation marks).
An undefined $value value will be returned as the
string NULL (without single quotation marks) to match
how NULLs are represented in SQL.
If $data_type is supplied, it is used to try to
determine the required quoting behavior by using the
information returned by the type_info entry elsewhere
in this document. As a special case, the standard
numeric types are optimized to return $value without
calling type_info.
Quote will probably not be able to deal with all
possible input (such as binary data or data containing
newlines), and is not related in any way with escaping
or quoting shell meta-characters. There is no need to
quote values being used with the section on
/"Placeholders and Bind Values.
quote_identifier
$sql = $dbh->quote_identifier( $name );
$sql = $dbh->quote_identifier( $name1, $name2, $name3, \%attr );
Quote an identifier (table name etc.) for use in an
SQL statement, by escaping any special characters
(such as double quotation marks) it contains and
adding the required type of outer quotation marks.
Undefined names are ignored and the remainder are
quoted and then joined together, typically with a dot
(.) character. For example:
$id = $dbh->quote_identifier( undef, 'Her schema', 'My table' );
would, for most database types, return "Her
schema"."My table" (including all the double quotation
marks).
If three names are supplied then the first is assumed
to be a catalog name and special rules may be applied
based on what the get_info entry elsewhere in this
documentreturns for SQL_CATALOG_NAME_SEPARATOR (41)
and SQL_CATALOG_LOCATION (114). For example, for
Oracle:
$id = $dbh->quote_identifier( 'link', 'schema', 'table' );
would return "schema"."table"@"link".
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Database Handle Attributes
This section describes attributes specific to database
handles.
Changes to these database handle attributes do not affect
any other existing or future database handles.
Attempting to set or get the value of an unknown attribute
is fatal, except for private driver-specific attributes
(which all have names starting with a lowercase letter).
Example:
$h->{AutoCommit} = ...; # set/write
... = $h->{AutoCommit}; # get/read
AutoCommit (boolean)
If true, then database changes cannot be rolled-back
(undone). If false, then database changes
automatically occur within a "transaction", which must
either be committed or rolled back using the commit or
rollback methods.
Drivers should always default to AutoCommit mode (an
unfortunate choice largely forced on the DBI by ODBC
and JDBC conventions.)
Attempting to set AutoCommit to an unsupported value
is a fatal error. This is an important feature of the
DBI. Applications that need full transaction behavior
can set $dbh->{AutoCommit} = 0 (or set AutoCommit to 0
via the connect entry elsewhere in this document)
without having to check that the value was assigned
successfully.
For the purposes of this description, we can divide
databases into three categories:
Databases which don't support transactions at all.
Databases in which a transaction is always active.
Databases in which a transaction must be explicitly started (C<'BEGIN WORK'>).
* Databases which don't support transactions at all
For these databases, attempting to turn AutoCommit off
is a fatal error. commit and rollback both issue
warnings about being ineffective while AutoCommit is
in effect.
* Databases in which a transaction is always active
These are typically mainstream commercial relational
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databases with "ANSI standard" transaction behavior.
If AutoCommit is off, then changes to the database
won't have any lasting effect unless the commit entry
elsewhere in this documentis called (but see also the
disconnect entry elsewhere in this document). If the
rollback entry elsewhere in this documentis called
then any changes since the last commit are undone.
If AutoCommit is on, then the effect is the same as if
the DBI called commit automatically after every
successful database operation. In other words, calling
commit or rollback explicitly while AutoCommit is on
would be ineffective because the changes would have
already been commited.
Changing AutoCommit from off to on should issue a the
commit entry elsewhere in this documentin most
drivers.
Changing AutoCommit from on to off should have no
immediate effect.
For databases which don't support a specific auto-
commit mode, the driver has to commit each statement
automatically using an explicit COMMIT after it
completes successfully (and roll it back using an
explicit ROLLBACK if it fails). The error information
reported to the application will correspond to the
statement which was executed, unless it succeeded and
the commit or rollback failed.
* Databases in which a transaction must be explicitly
started
For these databases, the intention is to have them act
like databases in which a transaction is always active
(as described above).
To do this, the DBI driver will automatically begin a
transaction when AutoCommit is turned off (from the
default "on" state) and will automatically begin
another transaction after a the commit entry elsewhere
in this documentor the rollback entry elsewhere in
this document. In this way, the application does not
have to treat these databases as a special case.
See the commit, disconnect, and Transactions entries
elsewhere in this documentfor other important notes
about transactions.
Driver (handle)
Holds the handle of the parent driver. The only
recommended use for this is to find the name of the
driver using:
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$dbh->{Driver}->{Name}
Name (string)
Holds the "name" of the database. Usually (and
recommended to be) the same as the
"dbi:DriverName:..." string used to connect to the
database, but with the leading "dbi:DriverName:"
removed.
Statement (string, read-only)
Returns the statement string passed to the most recent
the prepare entry elsewhere in this documentmethod
called in this database handle, even if that method
failed. This is especially useful where RaiseError is
enabled and the exception handler checks $@ and sees
that a 'prepare' method call failed.
RowCacheSize (integer)
A hint to the driver indicating the size of the local
row cache that the application would like the driver
to use for future SELECT statements. If a row cache
is not implemented, then setting RowCacheSize is
ignored and getting the value returns undef.
Some RowCacheSize values have special meaning, as
follows:
0 - Automatically determine a reasonable cache size for each C<SELECT>
1 - Disable the local row cache
>1 - Cache this many rows
<0 - Cache as many rows that will fit into this much memory for each C<SELECT>.
Note that large cache sizes may require a very large
amount of memory (cached rows * maximum size of row).
Also, a large cache will cause a longer delay not only
for the first fetch, but also whenever the cache needs
refilling.
See also the the RowsInCache entry elsewhere in this
documentstatement handle attribute.
DBI STATEMENT HANDLE OBJECTS
This section lists the methods and attributes associated
with DBI statement handles.
Statement Handle Methods
The DBI defines the following methods for use on DBI
statement handles:
bind_param
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$rc = $sth->bind_param($p_num, $bind_value) or die $sth->errstr;
$rv = $sth->bind_param($p_num, $bind_value, \%attr) or ...
$rv = $sth->bind_param($p_num, $bind_value, $bind_type) or ...
The bind_param method can be used to bind a value with
a placeholder embedded in the prepared statement.
Placeholders are indicated with question mark
character (?). For example:
$dbh->{RaiseError} = 1; # save having to check each method call
$sth = $dbh->prepare("SELECT name, age FROM people WHERE name LIKE ?");
$sth->bind_param(1, "John%"); # placeholders are numbered from 1
$sth->execute;
DBI::dump_results($sth);
Note that the ? is not enclosed in quotation marks,
even when the placeholder represents a string. Some
drivers also allow placeholders like :name and :n
(e.g., :1, :2, and so on) in addition to ?, but their
use is not portable. Undefined bind values or undef
can be used to indicate null values.
Some drivers do not support placeholders.
With most drivers, placeholders can't be used for any
element of a statement that would prevent the database
server from validating the statement and creating a
query execution plan for it. For example:
"SELECT name, age FROM ?" # wrong (will probably fail)
"SELECT name, ? FROM people" # wrong (but may not 'fail')
Also, placeholders can only represent single scalar
values. For example, the following statement won't
work as expected for more than one value:
"SELECT name, age FROM people WHERE name IN (?)" # wrong
Data Types for Placeholders
The \%attr parameter can be used to hint at the data
type the placeholder should have. Typically, the
driver is only interested in knowing if the
placeholder should be bound as a number or a string.
$sth->bind_param(1, $value, { TYPE => SQL_INTEGER });
As a short-cut for this common case, the data type can
be passed directly, in place of the \%attr hash
reference. This example is equivalent to the one
above:
$sth->bind_param(1, $value, SQL_INTEGER);
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The TYPE value indicates the standard (non-driver-
specific) type for this parameter. To specify the
driver-specific type, the driver may support a driver-
specific attribute, such as { ora_type => 97 }. The
data type for a placeholder cannot be changed after
the first bind_param call. However, it can be left
unspecified, in which case it defaults to the previous
value.
The SQL_INTEGER and other related constants can be
imported using
use DBI qw(:sql_types);
See the section on /"DBI Constants for more
information.
Perl only has string and number scalar data types. All
database types that aren't numbers are bound as
strings and must be in a format the database will
understand.
As an alternative to specifying the data type in the
bind_param call, you can let the driver pass the value
as the default type (VARCHAR). You can then use an
SQL function to convert the type within the statement.
For example:
INSERT INTO price(code, price) VALUES (?, CONVERT(MONEY,?))
The CONVERT function used here is just an example. The
actual function and syntax will vary between different
databases and is non-portable.
See also the section on /"Placeholders and Bind Values
for more information.
bind_param_inout
$rc = $sth->bind_param_inout($p_num, \$bind_value, $max_len) or die $sth->errstr;
$rv = $sth->bind_param_inout($p_num, \$bind_value, $max_len, \%attr) or ...
$rv = $sth->bind_param_inout($p_num, \$bind_value, $max_len, $bind_type) or ...
This method acts like the bind_param entry elsewhere
in this documentbut also enables values to be updated
by the statement. The statement is typically a call to
a stored procedure. The $bind_value must be passed as
a reference to the actual value to be used.
Note that unlike the bind_param entry elsewhere in
this documentthe $bind_value variable is not read when
bind_param_inout is called. Instead, the value in the
variable is read at the time the execute entry
elsewhere in this documentis called.
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The additional $max_len parameter specifies the
minimum amount of memory to allocate to $bind_value
for the new value. If the value returned from the
database is too big to fit, then the execution should
fail. If unsure what value to use, pick a generous
length, i.e., a length larger than the longest value
that would ever be returned. The only cost of using a
larger value than needed is wasted memory.
It is expected that few drivers will support this
method. The only driver currently known to do so is
DBD::Oracle (DBD::ODBC may support it in a future
release). Therefore it should not be used for database
independent applications.
Undefined values or undef are used to indicate null
values. See also the section on /"Placeholders and
Bind Values for more information.
execute
$rv = $sth->execute or die $sth->errstr;
$rv = $sth->execute(@bind_values) or die $sth->errstr;
Perform whatever processing is necessary to execute
the prepared statement. An undef is returned if an
error occurs. A successful execute always returns
true regardless of the number of rows affected, even
if it's zero (see below). It is always important to
check the return status of execute (and most other DBI
methods) for errors.
For a non-SELECT statement, execute returns the number
of rows affected, if known. If no rows were affected,
then execute returns "0E0", which Perl will treat as 0
but will regard as true. Note that it is not an error
for no rows to be affected by a statement. If the
number of rows affected is not known, then execute
returns -1.
For SELECT statements, execute simply "starts" the
query within the database engine. Use one of the fetch
methods to retreive the data after calling execute.
The execute method does not return the number of rows
that will be returned by the query (because most
databases can't tell in advance), it simply returns a
true value.
If any arguments are given, then execute will
effectively call the bind_param entry elsewhere in
this documentfor each value before executing the
statement. Values bound in this way are usually
treated as SQL_VARCHAR types unless the driver can
determine the correct type (which is rare), or unless
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bind_param (or bind_param_inout) has already been used
to specify the type.
fetchrow_arrayref
$ary_ref = $sth->fetchrow_arrayref;
$ary_ref = $sth->fetch; # alias
Fetches the next row of data and returns a reference
to an array holding the field values. Null fields are
returned as undef values in the array. This is the
fastest way to fetch data, particularly if used with
$sth->bind_columns.
If there are no more rows or if an error occurs, then
fetchrow_arrayref returns an undef. You should check
$sth->err afterwards (or use the RaiseError attribute)
to discover if the undef returned was due to an error.
Note that the same array reference is returned for
each fetch, so don't store the reference and then use
it after a later fetch. Also, the elements of the
array are also reused for each row, so take care if
you want to take a reference to an element. See also
the bind_columns entry elsewhere in this document.
fetchrow_array
@ary = $sth->fetchrow_array;
An alternative to fetchrow_arrayref. Fetches the next
row of data and returns it as a list containing the
field values. Null fields are returned as undef
values in the list.
If there are no more rows or if an error occurs, then
fetchrow_array returns an empty list. You should check
$sth->err afterwards (or use the RaiseError attribute)
to discover if the empty list returned was due to an
error.
In a scalar context, fetchrow_array returns the value
of the first field. An undef is returned if there are
no more rows or if an error occurred. Since that undef
can't be distinguished from an undef returned because
the first field value was NULL, you should exercise
some caution if you use fetchrow_array in a scalar
context.
fetchrow_hashref
$hash_ref = $sth->fetchrow_hashref;
$hash_ref = $sth->fetchrow_hashref($name);
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An alternative to fetchrow_arrayref. Fetches the next
row of data and returns it as a reference to a hash
containing field name and field value pairs. Null
fields are returned as undef values in the hash.
If there are no more rows or if an error occurs, then
fetchrow_hashref returns an undef. You should check
$sth->err afterwards (or use the RaiseError attribute)
to discover if the undef returned was due to an error.
The optional $name parameter specifies the name of the
statement handle attribute. For historical reasons it
defaults to "NAME", however using either "NAME_lc" or
"NAME_uc" is recomended for portability.
The keys of the hash are the same names returned by
$sth->{$name}. If more than one field has the same
name, there will only be one entry in the returned
hash for those fields.
Because of the extra work fetchrow_hashref and Perl
have to perform, it is not as efficient as
fetchrow_arrayref or fetchrow_array.
Currently, a new hash reference is returned for each
row. This will change in the future to return the
same hash ref each time, so don't rely on the current
behaviour.
fetchall_arrayref
$tbl_ary_ref = $sth->fetchall_arrayref;
$tbl_ary_ref = $sth->fetchall_arrayref( $columns_array_ref );
$tbl_ary_ref = $sth->fetchall_arrayref( $columns_hash_ref );
The fetchall_arrayref method can be used to fetch all
the data to be returned from a prepared and executed
statement handle. It returns a reference to an array
that contains one reference per row.
If there are no rows to return, fetchall_arrayref
returns a reference to an empty array. If an error
occurs, fetchall_arrayref returns the data fetched
thus far, which may be none. You should check
$sth->err afterwards (or use the RaiseError attribute)
to discover if the data is complete or was truncated
due to an error.
When passed an array reference, fetchall_arrayref uses
the fetchrow_arrayref entry elsewhere in this
documentto fetch each row as an array ref. If the
parameter array is not empty then it is used as a
slice to select individual columns by perl array index
number (starting at 0, unlike column and parameter
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numbers which start at 1).
With no parameters, fetchall_arrayref acts as if
passed an empty array ref.
When passed a hash reference, fetchall_arrayref uses
the fetchrow_hashref entry elsewhere in this
documentto fetch each row as a hash reference. If the
parameter hash is empty then fetchrow_hashref is
simply called in a tight loop and the keys in the
hashes have whatever name lettercase is returned by
default from fetchrow_hashref. (See the
FetchHashKeyName entry elsewhere in this
documentattribute.)
If the parameter hash is not empty, then it is used as
a slice to select individual columns by name. The
values of the hash should be set to 1. The key names
of the returned hashes match the letter case of the
names in the parameter hash, regardless of the the
FetchHashKeyName entry elsewhere in this
documentattribute.
For example, to fetch just the first column of every
row:
$tbl_ary_ref = $sth->fetchall_arrayref([0]);
To fetch the second to last and last column of every
row:
$tbl_ary_ref = $sth->fetchall_arrayref([-2,-1]);
To fetch all fields of every row as a hash ref:
$tbl_ary_ref = $sth->fetchall_arrayref({});
To fetch only the fields called "foo" and "bar" of
every row as a hash ref (with keys named "foo" and
"BAR"):
$tbl_ary_ref = $sth->fetchall_arrayref({ foo=>1, BAR=>1 });
The first two examples return a reference to an array
of array refs. The third and forth return a reference
to an array of hash refs.
fetchall_hashref
$hash_ref = $dbh->fetchall_hashref($key_field);
The fetchall_hashref method can be used to fetch all
the data to be returned from a prepared and executed
statement handle. It returns a reference to a hash
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that contains, at most, one entry per row.
If there are no rows to return, fetchall_hashref
returns a reference to an empty hash. If an error
occurs, fetchall_hashref returns the data fetched thus
far, which may be none. You should check $sth->err
afterwards (or use the RaiseError attribute) to
discover if the data is complete or was truncated due
to an error.
The $key_field parameter provides the name of the
field that holds the value to be used for the key for
the returned hash. For example:
$dbh->{FetchHashKeyName} = 'NAME_lc';
$sth = $dbh->prepare("SELECT FOO, BAR, ID, NAME, BAZ FROM TABLE");
$hash_ref = $sth->fetchall_hashref('id');
print "Name for id 42 is $hash_ref->{42}->{name}\n";
The $key_field parameter can also be specified as an
integer column number (counting from 1). If
$key_field doesn't match any column in the statement,
as a name first then as a number, then an error is
returned.
This method is normally used only where the key field
value for each row is unique. If multiple rows are
returned with the same value for the key field then
later rows overwrite earlier ones.
finish
$rc = $sth->finish;
Indicates that no more data will be fetched from this
statement handle before it is either executed again or
destroyed. The finish method is rarely needed, but
can sometimes be helpful in very specific situations
to allow the server to free up resources (such as sort
buffers).
When all the data has been fetched from a SELECT
statement, the driver should automatically call finish
for you. So you should not normally need to call it
explicitly. (Adding calls to finish after each fetch
loop is a common mistake, don't do it, it can mask
other problems.)
Consider a query like:
SELECT foo FROM table WHERE bar=? ORDER BY foo
where you want to select just the first (smallest)
"foo" value from a very large table. When executed,
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the database server will have to use temporary buffer
space to store the sorted rows. If, after executing
the handle and selecting one row, the handle won't be
re-executed for some time and won't be destroyed, the
finish method can be used to tell the server that the
buffer space can be freed.
Calling finish resets the the Active entry elsewhere
in this documentattribute for the statement. It may
also make some statement handle attributes (such as
NAME and TYPE) unavailable if they have not already
been accessed (and thus cached).
The finish method does not affect the transaction
status of the database connection. It has nothing to
do with transactions. It's mostly an internal
"housekeeping" method that is rarely needed. There's
no need to call finish if you're about to destroy or
re-execute the statement handle. See also the
disconnect entry elsewhere in this document and the
the Active entry elsewhere in this documentattribute.
The finish method should have been called
cancel_select.
rows
$rv = $sth->rows;
Returns the number of rows affected by the last row
affecting command, or -1 if the number of rows is not
known or not available.
Generally, you can only rely on a row count after a
non-SELECT execute (for some specific operations like
UPDATE and DELETE), or after fetching all the rows of
a SELECT statement.
For SELECT statements, it is generally not possible to
know how many rows will be returned except by fetching
them all. Some drivers will return the number of rows
the application has fetched so far, but others may
return -1 until all rows have been fetched. So use of
the rows method or $DBI::rows with SELECT statements
is not recommended.
One alternative method to get a row count for a SELECT
is to execute a "SELECT COUNT(*) FROM ..." SQL
statement with the same "..." as your query and then
fetch the row count from that.
bind_col
$rc = $sth->bind_col($column_number, \$var_to_bind);
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Binds an output column (field) of a SELECT statement
to a Perl variable. See bind_columns below for an
example. Note that column numbers count up from 1.
Whenever a row is fetched from the database, the
corresponding Perl variable is automatically updated.
There is no need to fetch and assign the values
manually. The binding is performed at a very low
level using Perl aliasing so there is no extra copying
taking place. This makes using bound variables very
efficient.
For maximum portability between drivers, bind_col
should be called after execute. This restriction may
be removed in a later version of the DBI.
You do not need to bind output columns in order to
fetch data, but it can be useful for some applications
which need either maximum performance or greater
clarity of code. The the bind_param entry elsewhere
in this documentmethod performs a similar but opposite
function for input variables.
bind_columns
$rc = $sth->bind_columns(@list_of_refs_to_vars_to_bind);
Calls the bind_col entry elsewhere in this documentfor
each column of the SELECT statement. The bind_columns
method will die if the number of references does not
match the number of fields.
For maximum portability between drivers, bind_columns
should be called after execute.
For example:
$dbh->{RaiseError} = 1; # do this, or check every call for errors
$sth = $dbh->prepare(q{ SELECT region, sales FROM sales_by_region });
$sth->execute;
my ($region, $sales);
# Bind Perl variables to columns:
$rv = $sth->bind_columns(\$region, \$sales);
# you can also use Perl's \(...) syntax (see perlref docs):
# $sth->bind_columns(\($region, $sales));
# Column binding is the most efficient way to fetch data
while ($sth->fetch) {
print "$region: $sales\n";
}
For compatibility with old scripts, the first
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parameter will be ignored if it is undef or a hash
reference.
Here's a more fancy example that binds columns to the
values inside a hash (thanks to H.Merijn Brand):
$sth->execute;
my %row;
$sth->bind_columns( \( @row{ @{$sth->{NAME_lc} } } ));
while ($sth->fetch) {
print "$row{region}: $row{sales}\n";
}
dump_results
$rows = $sth->dump_results($maxlen, $lsep, $fsep, $fh);
Fetches all the rows from $sth, calls DBI::neat_list
for each row, and prints the results to $fh (defaults
to STDOUT) separated by $lsep (default "\n"). $fsep
defaults to ", " and $maxlen defaults to 35.
This method is designed as a handy utility for
prototyping and testing queries. Since it uses the
neat_list entry elsewhere in this documentto format
and edit the string for reading by humans, it is not
recomended for data transfer applications.
Statement Handle Attributes
This section describes attributes specific to statement
handles. Most of these attributes are read-only.
Changes to these statement handle attributes do not affect
any other existing or future statement handles.
Attempting to set or get the value of an unknown attribute
is fatal, except for private driver specific attributes
(which all have names starting with a lowercase letter).
Example:
... = $h->{NUM_OF_FIELDS}; # get/read
Note that some drivers cannot provide valid values for
some or all of these attributes until after $sth->execute
has been called.
See also the finish entry elsewhere in this documentto
learn more about the effect it may have on some
attributes.
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NUM_OF_FIELDS (integer, read-only)
Number of fields (columns) the prepared statement will
return. Non-SELECT statements will have NUM_OF_FIELDS
== 0.
NUM_OF_PARAMS (integer, read-only)
The number of parameters (placeholders) in the
prepared statement. See SUBSTITUTION VARIABLES below
for more details.
NAME (array-ref, read-only)
Returns a reference to an array of field names for
each column. The names may contain spaces but should
not be truncated or have any trailing space. Note that
the names have the letter case (upper, lower or mixed)
as returned by the driver being used. Portable
applications should use the NAME_lc entry elsewhere in
this documentor the NAME_uc entry elsewhere in this
document.
print "First column name: $sth->{NAME}->[0]\n";
NAME_lc (array-ref, read-only)
Like the NAME entry elsewhere in this documentbut
always returns lowercase names.
NAME_uc (array-ref, read-only)
Like the NAME entry elsewhere in this documentbut
always returns uppercase names.
NAME_hash (hash-ref, read-only)
NAME_lc_hash (hash-ref, read-only)
NAME_uc_hash (hash-ref, read-only)
The NAME_hash, NAME_lc_hash, and NAME_uc_hash
attributes return column name information as a
reference to a hash.
The keys of the hash are the names of the columns.
The letter case of the keys corresponds to the letter
case returned by the NAME, NAME_lc, and NAME_uc
attributes respectively (as described above).
The value of each hash entry is the perl index number
of the corresponding column (counting from 0). For
example:
$sth = $dbh->prepare("select Id, Name from table");
$sth->execute;
@row = $sth->fetchrow_array;
print "Name $row[ $sth->{NAME_lc_hash}{name} ]\n";
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TYPE (array-ref, read-only)
Returns a reference to an array of integer values for
each column. The value indicates the data type of the
corresponding column.
The values correspond to the international standards
(ANSI X3.135 and ISO/IEC 9075) which, in general
terms, means ODBC. Driver-specific types that don't
exactly match standard types should generally return
the same values as an ODBC driver supplied by the
makers of the database. That might include private
type numbers in ranges the vendor has officially
registered with the ISO working group:
ftp://sqlstandards.org/SC32/SQL_Registry/
Where there's no vendor-supplied ODBC driver to be
compatible with, the DBI driver can use type numbers
in the range that is now officially reserved for use
by the DBI: -9999 to -9000.
All possible values for TYPE should have at least one
entry in the output of the type_info_all method (see
the type_info_all entry elsewhere in this document).
PRECISION (array-ref, read-only)
Returns a reference to an array of integer values for
each column. For non-numeric columns, the value
generally refers to either the maximum length or the
defined length of the column. For numeric columns,
the value refers to the maximum number of significant
digits used by the data type (without considering a
sign character or decimal point). Note that for
floating point types (REAL, FLOAT, DOUBLE), the
"display size" can be up to 7 characters greater than
the precision. (for the sign + decimal point + the
letter E + a sign + 2 or 3 digits).
SCALE (array-ref, read-only)
Returns a reference to an array of integer values for
each column. NULL (undef) values indicate columns
where scale is not applicable.
NULLABLE (array-ref, read-only)
Returns a reference to an array indicating the
possibility of each column returning a null. Possible
values are 0 (or an empty string) = no, 1 = yes, 2 =
unknown.
print "First column may return NULL\n" if $sth->{NULLABLE}->[0];
CursorName (string, read-only)
Returns the name of the cursor associated with the
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statement handle, if available. If not available or if
the database driver does not support the "where
current of ..." SQL syntax, then it returns undef.
Statement (string, read-only)
Returns the statement string passed to the the prepare
entry elsewhere in this documentmethod.
RowsInCache (integer, read-only)
If the driver supports a local row cache for SELECT
statements, then this attribute holds the number of
un-fetched rows in the cache. If the driver doesn't,
then it returns undef. Note that some drivers pre-
fetch rows on execute, whereas others wait till the
first fetch.
See also the the RowCacheSize entry elsewhere in this
documentdatabase handle attribute.
FURTHER INFORMATION
Transactions
Transactions are a fundamental part of any robust database
system. They protect against errors and database
corruption by ensuring that sets of related changes to the
database take place in atomic (indivisible, all-or-
nothing) units.
This section applies to databases that support
transactions and where AutoCommit is off. See the
AutoCommit entry elsewhere in this documentfor details of
using AutoCommit with various types of databases.
The recommended way to implement robust transactions in
Perl applications is to use RaiseError and eval { ... }
(which is very fast, unlike eval "..."). For example:
$dbh->{AutoCommit} = 0; # enable transactions, if possible
$dbh->{RaiseError} = 1;
eval {
foo(...) # do lots of work here
bar(...) # including inserts
baz(...) # and updates
$dbh->commit; # commit the changes if we get this far
};
if ($@) {
warn "Transaction aborted because $@";
$dbh->rollback; # undo the incomplete changes
# add other application on-error-clean-up code here
}
If the RaiseError attribute is not set, then DBI calls
would need to be manually checked for errors, typically
like this:
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$h->method(@args) or die $h->errstr;
With RaiseError set, the DBI will automatically die if any
DBI method call on that handle (or a child handle) fails,
so you don't have to test the return value of each method
call. See the RaiseError entry elsewhere in this
documentfor more details.
A major advantage of the eval approach is that the
transaction will be properly rolled back if any code (not
just DBI calls) in the inner application dies for any
reason. The major advantage of using the $h->{RaiseError}
attribute is that all DBI calls will be checked
automatically. Both techniques are strongly recommended.
After calling commit or rollback many drivers will not let
you fetch from a previously active SELECT statement handle
that's a child of the same database handle. A typical way
round this is to connect the the database twice and use
one connection for SELECT statements.
Handling BLOB / LONG / Memo Fields
Many databases support "blob" (binary large objects),
"long", or similar datatypes for holding very long strings
or large amounts of binary data in a single field. Some
databases support variable length long values over
2,000,000,000 bytes in length.
Since values of that size can't usually be held in memory,
and because databases can't usually know in advance the
length of the longest long that will be returned from a
SELECT statement (unlike other data types), some special
handling is required.
In this situation, the value of the $h->{LongReadLen}
attribute is used to determine how much buffer space to
allocate when fetching such fields. The $h->{LongTruncOk}
attribute is used to determine how to behave if a fetched
value can't fit into the buffer.
When trying to insert long or binary values, placeholders
should be used since there are often limits on the maximum
size of an INSERT statement and the the quote entry
elsewhere in this documentmethod generally can't cope with
binary data. See the section on /Placeholders and Bind
Values.
Simple Examples
Here's a complete example program to select and fetch some
data:
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my $dbh = DBI->connect("dbi:DriverName:db_name", $user, $password)
or die "Can't connect to $data_source: $DBI::errstr";
my $sth = $dbh->prepare( q{
SELECT name, phone
FROM mytelbook
}) or die "Can't prepare statement: $DBI::errstr";
my $rc = $sth->execute
or die "Can't execute statement: $DBI::errstr";
print "Query will return $sth->{NUM_OF_FIELDS} fields.\n\n";
print "Field names: @{ $sth->{NAME} }\n";
while (($name, $phone) = $sth->fetchrow_array) {
print "$name: $phone\n";
}
# check for problems which may have terminated the fetch early
die $sth->errstr if $sth->err;
$dbh->disconnect;
Here's a complete example program to insert some data from
a file. (This example uses RaiseError to avoid needing to
check each call).
my $dbh = DBI->connect("dbi:DriverName:db_name", $user, $password, {
RaiseError => 1, AutoCommit => 0
});
my $sth = $dbh->prepare( q{
INSERT INTO table (name, phone) VALUES (?, ?)
});
open FH, "<phone.csv" or die "Unable to open phone.csv: $!";
while (<FH>) {
chomp;
my ($name, $phone) = split /,/;
$sth->execute($name, $phone);
}
close FH;
$dbh->commit;
$dbh->disconnect;
Here's how to convert fetched NULLs (undefined values)
into empty strings:
while($row = $sth->fetchrow_arrayref) {
# this is a fast and simple way to deal with nulls:
foreach (@$row) { $_ = '' unless defined }
print "@$row\n";
}
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The q{...} style quoting used in these examples avoids
clashing with quotes that may be used in the SQL
statement. Use the double-quote like qq{...} operator if
you want to interpolate variables into the string. See
the section on Quote and Quote-like Operators in the
perlop manpage for more details.
Threads and Thread Safety
Perl versions 5.004_50 and later include optional
experimental support for multiple threads on many
platforms. If the DBI is built using a Perl that has
threads enabled then it will use a per-driver mutex to
ensure that only one thread is with a driver at any one
time. Please note that support for threads in Perl is
still experimental and is known to have some significant
problems. It's use is not recommended.
Signal Handling and Canceling Operations
The first thing to say is that signal handling in Perl is
currently not safe. There is always a small risk of Perl
crashing and/or core dumping when, or after, handling a
signal. (The risk was reduced with 5.004_04 but is still
present.)
The two most common uses of signals in relation to the DBI
are for canceling operations when the user types Ctrl-C
(interrupt), and for implementing a timeout using alarm()
and $SIG{ALRM}.
To assist in implementing these operations, the DBI
provides a cancel method for statement handles. The cancel
method should abort the current operation and is designed
to be called from a signal handler.
However, it must be stressed that: a) few drivers
implement this at the moment (the DBI provides a default
method that just returns undef); and b) even if
implemented, there is still a possibility that the
statement handle, and possibly the parent database handle,
will not be usable afterwards.
If cancel returns true, then it has successfully invoked
the database engine's own cancel function. If it returns
false, then cancel failed. If it returns undef, then the
database engine does not have cancel implemented.
Subclassing the DBI
DBI can be subclassed and extended just like any other
object oriented module. Before we talk about how to do
that, it's important to be clear about how the DBI classes
and how they work together.
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By default $dbh = DBI->connect(...) returns a $dbh blessed
into the DBI::db class. And the $dbh->prepare method
returns an $sth blessed into the DBI::st class (actually
it simply changes the last four characters of the calling
handle class to be ::st).
The leading 'DBI' is known as the 'root class' and the
extra '::db' or '::st' are the 'handle type suffixes'. If
you want to subclass the DBI you'll need to put your
overriding methods into the appropriate classes. For
example, if you want to use a root class of MySubDBI and
override the do(), prepare() and execute() methods, then
your do() and prepare() methods should be in the
MySubDBI::db class and the execute() method should be in
the MySubDBI::st class.
To setup the inheritance hierarchy the @ISA variable in
MySubDBI::db should include DBI::db and the @ISA variable
in MySubDBI::st should include DBI::st. The MySubDBI root
class itself isn't currently used for anything visible and
so, apart from setting @ISA to include DBI, it should be
left empty.
So, having put your overriding methods into the right
classes, and setup the inheritance hierarchy, how do you
get the DBI to use them? You have two choices, either a
static method call using the name of your subclass:
$dbh = MySubDBI->connect(...);
or specifying a RootClass attribute:
$dbh = DBI->connect(..., { RootClass => 'MySubDBI' });
The only difference between the two is that using an
explicit RootClass attribute will make the DBI
automatically attempt to load a module by that name (and
not complain if such a module can't be found). If both
forms are used then the attribute takes precedence.
Here's a brief example of a DBI subclass. A more thorough
example can be found in t/subclass.t in the DBI
distribution.
package MySubDBI;
use strict;
use DBI;
use vars qw(@ISA);
@ISA = qw(DBI);
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package MySubDBI::db;
use vars qw(@ISA);
@ISA = qw(DBI::db);
sub prepare {
my ($dbh, @args) = @_;
my $sth = $dbh->SUPER::prepare(@args)
or return;
$sth->{private_mysubdbi_info} = { foo => 'bar' };
return $sth;
}
package MySubDBI::st;
use vars qw(@ISA);
@ISA = qw(DBI::st);
sub fetch {
my ($sth, @args) = @_;
my $row = $sth->SUPER::fetch(@args)
or return;
do_something_magical_with_row_data($row)
or return $sth->set_err(1234, "The magic failed", undef, "fetch");
return $row;
}
When calling a SUPER::method that returns a handle, be
careful to check the return value before trying to do
other things with it in your overridden method. This is
especially important if you want to set a hash attribute
on the handle, as Perl's autovivification will bite you by
(in)conveniently creating an unblessed hashref, which your
method will then return with usually baffling results
later on. It's best to check right after the call and
return undef immediately on error, just like DBI would and
just like the example above.
If your method needs to record an error it should call the
set_err() method with the error code and error string, as
shown in the example above. The error code and error
string will be recorded in the handle and available via
$h->err and $DBI::errstr etc. The set_err() method always
returns an undef or empty list as approriate. Since your
method should nearly always return an undef or empty list
as soon as an error is detected it's handy to simply
return what set_err() returns, as shown in the example
above.
If the handle has RaiseError, PrintError, or HandleError
etc. set then the set_err() method will honour them. This
means that if RaiseError is set then set_err() won't
return in the normal way but will 'throw an exception'
that can be caught with an eval block.
You can stash private data into DBI handles via
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$h->{private_..._*}. See the entry under the section on
/ATTRIBUTES COMMON TO ALL HANDLES for info and important
caveats.
DEBUGGING
In addition to the the trace entry elsewhere in this
documentmethod, you can enable the same trace information
by setting the DBI_TRACE environment variable before
starting Perl.
On Unix-like systems using a Bourne-like shell, you can do
this easily on the command line:
DBI_TRACE=2 perl your_test_script.pl
If DBI_TRACE is set to a non-numeric value, then it is
assumed to be a file name and the trace level will be set
to 2 with all trace output appended to that file. If the
name begins with a number followed by an equal sign (=),
then the number and the equal sign are stripped off from
the name, and the number is used to set the trace level.
For example:
DBI_TRACE=1=dbitrace.log perl your_test_script.pl
See also the the trace entry elsewhere in this
documentmethod.
It can sometimes be handy to compare trace files from two
different runs of the same script. However using a tool
like diff doesn't work well because the trace file is full
of object addresses that may differ each run. Here's a
handy little command to strip those out:
perl -pe 's/\b0x[\da-f]{6,}/0xNNNN/gi; s/\b[\da-f]{6,}/<long number>/gi'
WARNING AND ERROR MESSAGES
Fatal Errors
Can't call method """"prepare"""" without a package or
object reference
The $dbh handle you're using to call prepare is
probably undefined because the preceding connect
failed. You should always check the return status of
DBI methods, or use the the RaiseError entry elsewhere
in this documentattribute.
Can't call method """"execute"""" without a package or
object reference
The $sth handle you're using to call execute is
probably undefined because the preceeding prepare
failed. You should always check the return status of
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DBI methods, or use the the RaiseError entry elsewhere
in this documentattribute.
DBI/DBD internal version mismatch
The DBD driver module was built with a different
version of DBI than the one currently being used. You
should rebuild the DBD module under the current
version of DBI.
(Some rare platforms require "static linking". On
those platforms, there may be an old DBI or DBD driver
version actually embedded in the Perl executable being
used.)
DBD driver has not implemented the AutoCommit attribute
The DBD driver implementation is incomplete. Consult
the author.
Can't [sg]et %s->{%s}: unrecognised attribute
You attempted to set or get an unknown attribute of a
handle. Make sure you have spelled the attribute name
correctly; case is significant (e.g., "Autocommit" is
not the same as "AutoCommit").
Warnings
Database handle destroyed without explicit disconnect
A $dbh handle went out of scope or the program ended
before the handle was disconnected from the database.
You should always explicitly call disconnect when you
are finished using a database handle. If using
transactions then you should also explicitly call
commit or rollback before disconnect.
DBI Handle cleared whilst still holding %d cached kids!
Most probably due to a DBI bug. Possibly a DBD driver
bug. Please report it.
DBI Handle cleared whilst still active!
Most probably due to a DBI bug. Possibly a DBD driver
bug. Please report it.
DBI Handle has uncleared implementors data
Most probably a DBD driver bug. Please report it.
DBI Handle has %d uncleared child handles
Most probably due to a DBI bug. Possibly a DBD driver
bug. Please report it.
SEE ALSO
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Driver and Database Documentation
Refer to the documentation for the DBD driver that you are
using.
Refer to the SQL Language Reference Manual for the
database engine that you are using.
Standards Reference Information
More detailed information about the semantics of certain
DBI methods that are based on ODBC and SQL/CLI standards
is available on-line via microsoft.com, for ODBC, and
www.jtc1sc32.org for the SQL/CLI standard:
DBI method ODBC function SQL/CLI Working Draft
---------- ------------- ---------------------
column_info SQLColumns Page 124
foreign_key_info SQLForeignKeys Page 163
get_info SQLGetInfo Page 214
primary_key_info SQLPrimaryKeys Page 254
table_info SQLTables Page 294
type_info SQLGetTypeInfo Page 239
For example, for ODBC information on SQLColumns you'd
visit:
http://msdn.microsoft.com/library/en-us/odbc/htm/odbcsqlcolumns.asp
If that URL ceases to work then use the MSDN search
facility at:
http://search.microsoft.com/us/dev/
and search for SQLColumns returns using the exact phrase
option. The link you want will probably just be called
SQLColumns and will be part of the Data Access SDK.
And for SQL/CLI standard information on SQLColumns you'd
read page 124 of the (very large) SQL/CLI Working Draft
available from:
http://www.jtc1sc32.org/sc32/jtc1sc32.nsf/Attachments/7E3B41486BD99C3488256B410064C877/$FILE/32N0744T.PDF
A hyperlinked, browsable version of the BNF syntax for
SQL92 (plus Oracle 7 SQL and PL/SQL) is available here:
http://cui.unige.ch/db-research/Enseignement/analyseinfo/SQL92/BNFindex.html
A BNF syntax for SQL3 is available here:
http://www.sqlstandards.org/SC32/WG3/Progression_Documents/Informal_working_drafts/iso-9075-2-1999.bnf
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Books and Journals
Programming the Perl DBI, by Alligator Descartes and Tim Bunce.
Programming Perl 2nd Ed. by Larry Wall, Tom Christiansen & Randal Schwartz.
Learning Perl by Randal Schwartz.
Dr Dobb's Journal, November 1996.
The Perl Journal, April 1997.
Perl Modules
Index of DBI related modules available from CPAN:
http://search.cpan.org/search?mode=module&query=DBIx%3A%3A
http://search.cpan.org/search?mode=doc&query=DBI
For a good comparison of RDBMS-OO mappers and some
OO-RDBMS mappers (including Class::DBI, Alzabo, and
DBIx::RecordSet in the former category and Tangram and
SPOPS in the latter) see the Perl Object-Oriented
Persistence project pages at:
http://poop.sourceforge.net
Manual Pages
the perl(1) manpage, the perlmod(1) manpage, the
perlbook(1) manpage
Mailing List
The dbi-users mailing list is the primary means of
communication among users of the DBI and its related
modules. For details send email to:
dbi-users-help@perl.org
There are typically between 700 and 900 messages per
month. You have to subscribe in order to be able to post.
However you can opt for a 'post-only' subscription.
Mailing list archives (of variable quality) are held at:
http://www.xray.mpe.mpg.de/mailing-lists/dbi/
http://groups.yahoo.com/group/dbi-users
http://www.bitmechanic.com/mail-archives/dbi-users/
http://marc.theaimsgroup.com/?l=perl-dbi&r=1&w=2
http://www.mail-archive.com/dbi-users%40perl.org/
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Assorted Related WWW Links
The DBI "Home Page":
http://dbi.perl.org/
Other DBI related links:
http://tegan.deltanet.com/~phlip/DBUIdoc.html
http://dc.pm.org/perl_db.html
http://wdvl.com/Authoring/DB/Intro/toc.html
http://www.hotwired.com/webmonkey/backend/tutorials/tutorial1.html
http://bumppo.net/lists/macperl/1999/06/msg00197.html
Other database related links:
http://www.jcc.com/sql_stnd.html
http://cuiwww.unige.ch/OSG/info/FreeDB/FreeDB.home.html
Commercial and Data Warehouse Links
http://www.dwinfocenter.org
http://www.datawarehouse.com
http://www.datamining.org
http://www.olapcouncil.org
http://www.idwa.org
http://www.knowledgecenters.org/dwcenter.asp
Recommended Perl Programming Links
http://language.perl.com/style/
FAQ
Please also read the DBI FAQ which is installed as a
DBI::FAQ module. You can use perldoc to read it by
executing the perldoc DBI::FAQ command.
AUTHORS
DBI by Tim Bunce. This pod text by Tim Bunce, J. Douglas
Dunlop, Jonathan Leffler and others. Perl by Larry Wall
and the perl5-porters.
COPYRIGHT
The DBI module is Copyright (c) 1994-2000 Tim Bunce.
England. All rights reserved.
You may distribute under the terms of either the GNU
General Public License or the Artistic License, as
specified in the Perl README file.
ACKNOWLEDGEMENTS
I would like to acknowledge the valuable contributions of
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the many people I have worked with on the DBI project,
especially in the early years (1992-1994). In no
particular order: Kevin Stock, Buzz Moschetti, Kurt
Andersen, Ted Lemon, William Hails, Garth Kennedy, Michael
Peppler, Neil S. Briscoe, Jeff Urlwin, David J. Hughes,
Jeff Stander, Forrest D Whitcher, Larry Wall, Jeff Fried,
Roy Johnson, Paul Hudson, Georg Rehfeld, Steve Sizemore,
Ron Pool, Jon Meek, Tom Christiansen, Steve Baumgarten,
Randal Schwartz, and a whole lot more.
Then, of course, there are the poor souls who have
struggled through untold and undocumented obstacles to
actually implement DBI drivers. Among their ranks are
Jochen Wiedmann, Alligator Descartes, Jonathan Leffler,
Jeff Urlwin, Michael Peppler, Henrik Tougaard, Edwin
Pratomo, Davide Migliavacca, Jan Pazdziora, Peter Haworth,
Edmund Mergl, Steve Williams, Thomas Lowery, and Phlip
Plumlee. Without them, the DBI would not be the practical
reality it is today. I'm also especially grateful to
Alligator Descartes for starting work on the "Programming
the Perl DBI" book and letting me jump on board.
Much of the DBI and DBD::Oracle was developed while I was
Technical Director (CTO) of the Paul Ingram Group
(www.ig.co.uk). So I'd especially like to thank Paul for
his generosity and vision in supporting this work for many
years.
TRANSLATIONS
A German translation of this manual (possibly slightly out
of date) is available, thanks to O'Reilly, at:
http://www.oreilly.de/catalog/perldbiger/
Some other translations:
http://cronopio.net/perl/ - Spanish
http://member.nifty.ne.jp/hippo2000/dbimemo.htm - Japanese
SUPPORT / WARRANTY
The DBI is free software. IT COMES WITHOUT WARRANTY OF ANY
KIND.
Commercial support for Perl and the DBI, DBD::Oracle and
Oraperl modules can be arranged via The Perl Clinic. For
more details visit:
http://www.perlclinic.com
For direct DBI and DBD::Oracle support, enhancement, and
related work I am available for consultancy on standard
commercial terms.
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TRAINING
References to DBI related training resources. No
recommendation implied.
http://www.treepax.co.uk/
http://www.keller.com/dbweb/
FREQUENTLY ASKED QUESTIONS
See the DBI FAQ for a more comprehensive list of FAQs. Use
the perldoc DBI::FAQ command to read it.
How fast is the DBI?
To measure the speed of the DBI and DBD::Oracle code, I
modified DBD::Oracle so you can set an attribute that will
cause the same row to be fetched from the row cache over
and over again (without involving Oracle code but
exercising *all* the DBI and DBD::Oracle code in the code
path for a fetch).
The results (on my lightly loaded old Sparc 10) fetching
50000 rows using:
1 while $csr->fetch;
were: one field: 5300 fetches per cpu second
(approx) ten fields: 4000 fetches per cpu second
(approx)
Obviously results will vary between platforms (newer
faster platforms can reach around 50000 fetches per
second), but it does give a feel for the maximum
performance: fast. By way of comparison, using the code:
1 while @row = $csr->fetchrow_array;
(fetchrow_array is roughly the same as ora_fetch) gives:
one field: 3100 fetches per cpu second (approx)
ten fields: 1000 fetches per cpu second (approx)
Notice the slowdown and the more dramatic impact of extra
fields. (The fields were all one char long. The impact
would be even bigger for longer strings.)
Changing that slightly to represent actually doing
something in Perl with the fetched data:
while(@row = $csr->fetchrow_array) {
$hash{++$i} = [ @row ];
}
gives: ten fields: 500 fetches per cpu second (approx)
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That simple addition has *halved* the performance.
I therefore conclude that DBI and DBD::Oracle overheads
are small compared with Perl language overheads (and
probably database overheads).
So, if you think the DBI or your driver is slow, try
replacing your fetch loop with just:
1 while $csr->fetch;
and time that. If that helps then point the finger at your
own code. If that doesn't help much then point the finger
at the database, the platform, the network etc. But think
carefully before pointing it at the DBI or your driver.
(Having said all that, if anyone can show me how to make
the DBI or drivers even more efficient, I'm all ears.)
Why doesn't my CGI script work right?
Read the information in the references below. Please do
not post CGI related questions to the dbi-users mailing
list (or to me).
http://www.perl.com/cgi-bin/pace/pub/doc/FAQs/cgi/perl-cgi-faq.html
http://www3.pair.com/webthing/docs/cgi/faqs/cgifaq.shtml
http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html
http://www.boutell.com/faq/
http://www.perl.com/perl/faq/
General problems and good ideas:
Use the CGI::ErrorWrap module.
Remember that many env vars won't be set for CGI scripts.
How can I maintain a WWW connection to a database?
For information on the Apache httpd server and the
mod_perl module see
http://perl.apache.org/
What about ODBC?
A DBD::ODBC module is available.
Does the DBI have a year 2000 problem?
No. The DBI has no knowledge or understanding of dates at
all.
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Individual drivers (DBD::*) may have some date handling
code but are unlikely to have year 2000 related problems
within their code. However, your application code which
uses the DBI and DBD drivers may have year 2000 related
problems if it has not been designed and written well.
See also the "Does Perl have a year 2000 problem?" section
of the Perl FAQ:
http://www.perl.com/CPAN/doc/FAQs/FAQ/PerlFAQ.html
OTHER RELATED WORK AND PERL MODULES
Apache::DBI by E.Mergl@bawue.de
To be used with the Apache daemon together with an
embedded Perl interpreter like mod_perl. Establishes a
database connection which remains open for the
lifetime of the HTTP daemon. This way the CGI connect
and disconnect for every database access becomes
superfluous.
JDBC Server by Stuart 'Zen' Bishop zen@bf.rmit.edu.au
The server is written in Perl. The client classes that
talk to it are of course in Java. Thus, a Java applet
or application will be able to comunicate via the JDBC
API with any database that has a DBI driver installed.
The URL used is in the form
jdbc:dbi://host.domain.etc:999/Driver/DBName. It
seems to be very similar to some commercial products,
such as jdbcKona.
Remote Proxy DBD support
As of DBI 1.02, a complete implementation of a
DBD::Proxy driver and the DBI::ProxyServer are part of
the DBI distribution.
SQL Parser
See also the SQL::Statement module, a very simple SQL
parser and engine, base of the DBD::CSV driver.
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