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);
@row_ary = $dbh->selectrow_array($statement);
$ary_ref = $dbh->selectrow_arrayref($statement);
$hash_ref = $dbh->selectrow_hashref($statement);
$sth = $dbh->prepare($statement);
$sth = $dbh->prepare_cached($statement);
$rc = $sth->bind_param($p_num, $bind_value);
$rc = $sth->bind_param($p_num, $bind_value, $bind_type);
$rc = $sth->bind_param($p_num, $bind_value, \%attr);
$rv = $sth->execute;
$rv = $sth->execute(@bind_values);
$rc = $sth->bind_param_array($p_num, $bind_values, \%attr);
$rv = $sth->execute_array(\%attr);
$rv = $sth->execute_array(\%attr, @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( $slice, $max_rows );
$hash_ref = $sth->fetchall_hashref( $key_field );
$rv = $sth->rows;
$rc = $dbh->begin_work;
$rc = $dbh->commit;
$rc = $dbh->rollback;
$quoted_string = $dbh->quote($string);
$rc = $h->err;
$str = $h->errstr;
$rv = $h->state;
$rc = $dbh->disconnect;
This synopsis above only lists the major methods.
If you have questions about DBI, you can get help from
the dbi-
mailing list.
You can get help on subscribing and using the list by emailing:
dbi-users-
Also worth a visit is the DBI home page at:
http://dbi.perl.org/
Before asking any questions, reread this document, consult the
archives and read the DBI FAQ. The archives are listed
at the end of this document and on the DBI home page.
The FAQ is installed as a DBI::FAQ module so
you can read it by executing perldoc DBI::FAQ.
To help you make the best use of the dbi-users mailing list,
and any other lists or forums you may use, I strongly
recommend that you read "How To Ask Questions The Smart Way"
by Eric Raymond:
http://www.tuxedo.org/~esr/faqs/smart-questions.html
This document often uses terms like references, objects,
methods. If you're not familar with those terms then it would
be a good idea to read at least the following perl manuals first:
perlreftut, perldsc, perllol, and perlboot.
Please note that Tim Bunce does not maintain the mailing lists or the
web page (generous volunteers do that). So please don't send mail
directly to him; he just doesn't have the time to answer questions
personally. The dbi-users mailing list has lots of experienced
people who should be able to help you if you need it. If you do email
Tim he's very likely to just forward it to the mailing list.
This is the DBI specification that corresponds to the DBI version 1.32
($Date: 2002/12/01 22:34:29 $).
The DBI is evolving at a steady pace, so it's good to check that
you have the latest copy.
The significant user-visible changes in each release are documented
in the DBI::Changes module so you can read them by executing
perldoc DBI::Changes.
Some DBI changes require changes in the drivers, but the drivers
can 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 API often use the DBIx::* namespace.
See /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.
|<- 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.
The following conventions are used in this document:
$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.
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 /connect to 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:
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 /AutoCommit is off):
$dbh->commit; # or call $dbh->rollback; to undo changes
Finally, when you have finished working with the data source, you should
/disconnect from it:
$dbh->disconnect;
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 parameter can only be used to pass
driver specific hints. In general, you can ignore \%attr parameters
or pass it as undef.
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:
ad_ DBD::AnyData
ado_ DBD::ADO
best_ DBD::BestWins
csv_ DBD::CSV
db2_ DBD::DB2
df_ DBD::DF
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
sql_ SQL::Statement (used by some drivers)
syb_ DBD::Sybase
tdat_ DBD::Teradata
tmpl_ DBD::Template
tmplss_ DBD::TemplateSS
tuber_ DBD::Tuber
uni_ DBD::Unify
xbase_ DBD::XBase
xl_ DBD::Excel
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 /prepare or
/do methods.
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.
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. Sadly, that more general
syntax doesn't work for Sybase and MS SQL Server. However on those two
servers the original "product_code = ?" syntax works for binding nulls.
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 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 /execute and /bind_param for 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 perlop/"Quote and Quote-like Operators" for more details.
See also the /bind_column method, 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.
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 /type_info, /type_info_all, and /bind_param methods
for possible uses.
Note that just because the DBI defines a named constant for a given
data type doesn't mean that drivers will support that data type.
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:
: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. (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:
$ENV{DBI_AUTOPROXY};dsn=$data_source
DBI_AUTOPROXY is typically set as "dbi:Proxy:hostname=...;port=...".
If $ENV{DBI_AUTOPROXY} doesn't begin with 'dbi:' then "dbi:Proxy:"
will be prepended to it first. 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 /AutoCommit and /PrintError for 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:
$dbh = DBI->connect($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 /connect, except that the database handle
returned is also
stored in a hash associated with the given parameters. If another call
is made to connect_cached with the same parameter values, then the
corresponding cached $dbh will be returned if it is still valid.
The cached database handle is replaced with a new connection if it
has been disconnected or if the ping method fails.
Note that the behavior of this method differs in several respects from the
behavior of presistent connections implemented by Apache::DBI.
Caching can be useful in some applications, but it can also cause
problems and should be used with care. The exact behavior of this
method is liable to change, so if you intend to use it in any production
applications you should discuss your needs on the dbi-users mailing list.
The cache can be accessed (and cleared) via the /CachedKids attribute.
available_drivers
-
@ary = DBI->available_drivers;
@ary = DBI->available_drivers($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 $driver is empty or undef, then the value of the
DBI_DRIVER environment variable is used.
The driver will be loaded if it hasn't been already. Note that if the
driver loading fails then it dies with an error message that
includes the string "install_driver" and the underlying problem.
Data sources are returned in a form suitable for passing to the
/connect method (that is, they will include the "dbi:$driver:" prefix).
Note that many drivers have no way of knowing what 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.
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 /neat function, 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
/DEBUGGING section
for information about the DBI_TRACE environment variable.
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 /trace output. It should
typically not be used for formatting values for database use.
(See also /quote.)
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.
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.
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 /rows method.
$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.
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 /err method
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 '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 /err method 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 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 /neat function, 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 /DEBUGGING for 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 enabled for $h or
for the DBI as a whole. Can also be called as DBI->trace_msg($msg).
See /trace.
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, $func_name) or die ...;
The func method can be used to call private non-standard and
non-portable methods implemented by the driver. Note that the function
name is given as the last argument.
It's also important to note that the func() method does not clear
a previous error ($DBI::err etc.) and it does not trigger automatic
error detection (RaiseError etc.) so you must check the return
status and/or $h->err to detect errors.
(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 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 /prepare_cached method. For a
driver handle, returns a reference to the cache (hash) of
database handles created by the /connect_cached method.
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.
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.
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)
-
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 die with 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 perlsub and perlref for more information
about closures.
It is possible for HandleError to alter the error message that
will be used by RaiseError and PrintError if it returns false.
It can do that by altering the value of $_[0]. This example appends
a stack trace to all errors and, unlike the previous example using
Carp::confess, this will work PrintError as well as RaiseError:
$h->{HandleError} = sub { $_[0]=Carp::longmess($_[0]); 0; };
It is also possible for HandleError to hide an error, to a limited
degree, by using /set_err to reset $DBI::err and $DBI::errstr,
and altering the return value of the failed method. For example:
$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(), do(), and the various select*() database handle methods.
(The exact format of the appended text is subject to change.)
If $h->{ParamValues} returns a hash reference of parameter
(placeholder) values then those are formatted and appened to the
end of the Statement text in the error message.
TraceLevel (integer, inherited) NEW
-
This attribute can be used as an alternative to the /trace method
to set the DBI trace level for a specific handle.
FetchHashKeyName (string, inherited)
-
This attribute is used to specify whether the fetchrow_hashref()
method should perform case conversion on the field names used for
the hash keys. For historical reasons it defaults to '
NAME' but
it is recommended to set it to 'NAME_lc' (convert to lower case)
or 'NAME_uc' (convert to upper case) according to your preference.
It can only be set for driver and database handles. 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.
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 /LongTruncOk for 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 /LongReadLen.
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 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 perlsec 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 TaintIn attribute,
for that statement handle, for the duration of the fetch loop.
TaintOut (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 most data fetched
from the database is considered tainted. This may change.
The attribute defaults to off, even if Perl is in taint mode.
See perlsec 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 TaintOut 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.
Taint (boolean, inherited)
-
This value is shortcut for /TaintIn and /TaintOut (it is also present
for backwards compatability).
Setting this attribute sets both /TaintIn and /TaintOut, and retrieving
it returns a true value if and only if /TaintIn and /TaintOut are
both set to true values.
Profile (inherited)
-
Enable collection and reporting of method call timing statistics.
See the DBI::Profile module documentation for much more detail.
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.
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 perlop/"Quote and Quote-like Operators" for more details.
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 /prepare, /execute and
/fetchrow_array into a single call. If called in a list context, 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 /RaiseError is not set, selectrow_array
will return an empty list.
If called in a scalar context for a statement handle that has more
than one column, it is undefined whether the driver will return
the value of the first column or the last. So don't do that.
Also, in a scalar context, an undef is returned if there are no
more rows or if an error occurred. That undef can't be distinguished
from an undef returned because the first field value was NULL.
For these reasons you should exercise some caution if you use
selectrow_array in a scalar context.
selectrow_arrayref
-
$ary_ref = $dbh->selectrow_arrayref($statement);
$ary_ref = $dbh->selectrow_arrayref($statement, \%attr);
$ary_ref = $dbh->selectrow_arrayref($statement, \%attr, @bind_values);
This utility method combines /prepare, /execute and
/fetchrow_arrayref into 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 /RaiseError is 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 /prepare, /execute and
/fetchrow_hashref into 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 /RaiseError is 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 /prepare, /execute and
/fetchall_arrayref into 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 /RaiseError is 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 /fetchall_arrayref method called by selectall_arrayref
supports a $max_rows parameter. You can specify a value for $max_rows
by including a 'MaxRows' attribute in \%attr.
The /fetchall_arrayref method called by selectall_arrayref
also 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 /prepare, /execute and
/fetchall_hashref into a single call. It returns a reference to 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 /RaiseError is not set,
selectall_hashref will return undef. If fetchrow_hashref fails and
/RaiseError is 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 /prepare, /execute, 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 /RaiseError is not set,
selectcol_arrayref will return undef. If fetch fails and
/RaiseError is 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
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 /execute method. See /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 /Placeholders). 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 /prepare except 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 cached $sth will be returned without contacting the
database server.
Here are some examples of prepare_cached:
sub insert_hash {
my ($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(",", ("?")
);
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);
}
Caveat emptor: This caching can be useful in some applications,
but it can also cause problems and should be used with care. Here
is a contrived case where caching would cause a significant problem:
my $sth = $dbh->prepare_cached('SELECT * FROM foo WHERE bar=?');
$sth->execute($bar);
while (my $data = $sth->fetchrow_hashref) {
my $sth2 = $dbh->prepare_cached('SELECT * FROM foo WHERE bar=?');
$sth2->execute($data->{bar});
while (my $data2 = $sth2->fetchrow_arrayref) {
do_stuff(...);
}
}
In this example, since both handles are preparing the exact same statement,
$sth2 will not be its own statement handle, but a duplicate of $sth
returned from the cache. The results will certainly not be what you expect.
Typically the the inner fetch loop will work normally, fetching all
the records and terminating when there are no more, but now $sth
is the same as $sth2 the outer fetch loop will also terminate.
The $allow_active parameter lets you adjust DBI's behavior when
prepare_cached is returning a statement handle that is still active.
There are three settings:
0: A warning will be generated, and finish will be called on
the statement handle before it is returned. This is the default
behavior if $allow_active is not passed.
1: finish will be called on the statement handle, but the
warning is suppressed.
2: DBI will not touch the statement handle before returning it.
You will need to check $sth->{Active} on the returned
statement handle and deal with it in your own code.
Because the cache used by prepare_cached() is keyed by all the
parameters, including any attributes passed, you can also avoid
this issue by doing something like:
my $sth = $dbh->prepare_cached("...", { dbi_dummy => __FILE__.__LINE__ });
which will ensure that prepare_cached only returns statements cached
by that line of code in that source file.
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 /Transactions in the /FURTHER INFORMATION section below.
rollback
-
$rc = $dbh->rollback or die $dbh->errstr;
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 /Transactions in the /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 /Transactions in the /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 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 /commit or /rollback
before disconnecting.
If you disconnect from a database while you still have active
statement handles (e.g., SELECT statement handles that may have
more data to fetch), you will get a warning. The warning may indicate
that a fetch loop terminated early, perhaps due to an uncaught error.
To avoid the warning call the finish method on the active handles.
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 /"Standards Reference Information" for more detailed information
about the information types and their meanings and possible return values.
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 constant 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 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:
- *
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('%', '', '');
- *
If the value of $schema is '%' and $catalog and $table are empty
strings, the result set contains a list of schema names.
-
- *
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).
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 /tables, /"Catalog Methods" and
/"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 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
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 /"Catalog Methods" and /"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.
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 /
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