SQL::Abstract::Limit(3pm)				User Contributed Perl Documentation				 SQL::Abstract::Limit(3pm)

NAME

       SQL::Abstract::Limit - portable LIMIT emulation

SYNOPSIS

	   use SQL::Abstract::Limit;

	   my $sql = SQL::Abstract::Limit->new( limit_dialect => 'LimitOffset' );;

	   # or autodetect from a DBI $dbh:
	   my $sql = SQL::Abstract::Limit->new( limit_dialect => $dbh );

	   # or from a Class::DBI class:
	   my $sql = SQL::Abstract::Limit->new( limit_dialect => 'My::CDBI::App' );

	   # or object:
	   my $obj = My::CDBI::App->retrieve( $id );
	   my $sql = SQL::Abstract::Limit->new( limit_dialect => $obj );

	   # generate SQL:
	   my ( $stmt, @bind ) = $sql->select( $table, @fields, \%where, @order, $limit, $offset );

	   # Then, use these in your DBI statements
	   my $sth = $dbh->prepare( $stmt );
	   $sth->execute( @bind );

	   # Just generate the WHERE clause (only available for some syntaxes)
	   my ( $stmt, @bind )	= $sql->where( \%where, @order, $limit, $offset );

DESCRIPTION

       Portability layer for LIMIT emulation.

       new( case => 'lower', cmp => 'like', logic => 'and', convert => 'upper', limit_dialect => 'Top' )
	   All settings are optional.

	   limit_dialect
		   Sets the default syntax model to use for emulating a "LIMIT $rows OFFSET $offset" clause. Default setting is "GenericSubQ". You
		   can still pass other syntax settings in method calls, this just sets the default. Possible values are:

		       LimitOffset     PostgreSQL, SQLite
		       LimitXY	       MySQL, MaxDB, anything that uses SQL::Statement
		       LimitYX	       SQLite (optional)
		       RowsTo	       InterBase/FireBird

		       Top	       SQL/Server, MS Access
		       RowNum	       Oracle
		       FetchFirst      DB2
		       Skip	       Informix
		       GenericSubQ     Sybase, plus any databases not recognised by this module

		       $dbh	       a DBI database handle

		       CDBI subclass
		       CDBI object

		       other DBI-based thing

		   The first group are implemented by appending a short clause to the end of the statement. The second group require more
		   intricate wrapping of the original statement in subselects.

		   You can pass a DBI database handle, and the module will figure out which dialect to use.

		   You can pass a Class::DBI subclass or object, and the module will find the $dbh and use it to find the dialect.

		   Anything else based on DBI can be easily added by locating the $dbh.  Patches or suggestions welcome.

	   Other options are described in SQL::Abstract.

       select( $table, @fields, $where, [ @order, [ $rows, [ $offset ], [ $dialect ] ] ] )
	   Same as "SQL::Abstract::select", but accepts additional $rows, $offset and $dialect parameters.

	   The $order parameter is required if $rows is specified.

	   The $fields parameter is required, but can be set to "undef", '' or '*' (all these get set to '*').

	   The $where parameter is also required. It can be a hashref or an arrayref, or "undef".

       where( [ $where, [ @order, [ $rows, [ $offset ], [ $dialect ] ] ] ] )
	   Same as "SQL::Abstract::where", but accepts additional $rows, $offset and $dialect parameters.

	   Some SQL dialects support syntaxes that can be applied as simple phrases tacked on to the end of the WHERE clause. These are:

	       LimitOffset
	       LimitXY
	       LimitYX
	       RowsTo

	   This method returns a modified WHERE clause, if the limit syntax is set to one of these options (either in the call to "where" or in
	   the constructor), and if $rows is passed in.

	   Dies via "croak" if you try to use it for other syntaxes.

	   $order is required if $rows is set.

	   $where is required if any other parameters are specified. It can be a hashref or an arrayref, or "undef".

	   Returns a regular "WHERE" clause if no limits are set.

       insert
       update
       delete
       values
       generate
	   See SQL::Abstract for these methods.

	   "update" and "delete" are not provided with any "LIMIT" emulation in this release, and no support is planned at the moment. But patches
	   would be welcome.

   Limit emulation
       The following dialects are available for emulating the LIMIT clause. In each case, $sql represents the SQL statement generated by
       "SQL::Abstract::select", minus the ORDER BY clause, e.g.

	   SELECT foo, bar FROM my_table WHERE some_conditions

       $sql_after_select represents $sql with the leading "SELECT" keyword removed.

       "order_cols_up" represents the sort column(s) and direction(s) specified in the "order" parameter.

       "order_cols_down" represents the opposite sort.

       "$last = $rows + $offset"

       LimitOffset
	   Syntax
		       $sql ORDER BY order_cols_up LIMIT $rows OFFSET $offset

		   or

		       $sql ORDER BY order_cols_up LIMIT $rows

		   if "$offset == 0".

	   Databases
		       PostgreSQL
		       SQLite

       LimitXY
	   Syntax
		       $sql ORDER BY order_cols_up LIMIT $offset, $rows

		   or

			$sql ORDER BY order_cols_up LIMIT $rows

		   if "$offset == 0".

	   Databases
		       MySQL

       LimitYX
	   Syntax
		       $sql ORDER BY order_cols_up LIMIT $rows, $offset

		   or

		       $sql ORDER BY order_cols_up LIMIT $rows

		   if "$offset == 0".

	   Databases
		       SQLite understands this syntax, or LimitOffset. If autodetecting the
			      dialect, it will be set to LimitOffset.

       RowsTo
	   Syntax
		       $sql ORDER BY order_cols_up ROWS $offset TO $last

	   Databases
		       InterBase
		       FireBird

       Top
	   Syntax
		       SELECT * FROM
		       (
			   SELECT TOP $rows * FROM
			   (
			       SELECT TOP $last $sql_after_select
			       ORDER BY order_cols_up
			   ) AS foo
			   ORDER BY order_cols_down
		       ) AS bar
		       ORDER BY order_cols_up

	   Databases
		       SQL/Server
		       MS Access

       RowNum
	   Syntax  Oracle numbers rows from 1, not zero, so here $offset has been incremented by 1.

		       SELECT * FROM
		       (
			   SELECT A.*, ROWNUM r FROM
			   (
			       $sql ORDER BY order_cols_up
			   ) A
			   WHERE ROWNUM <= $last
		       ) B
		       WHERE r >= $offset

	   Databases
		       Oracle

       FetchFirst
	   Syntax
		       SELECT * FROM (
			   SELECT * FROM (
			       $sql
			       ORDER BY order_cols_up
			       FETCH FIRST $last ROWS ONLY
			   ) foo
			   ORDER BY order_cols_down
			   FETCH FIRST $rows ROWS ONLY
		       ) bar
		       ORDER BY order_cols_up

	   Databases
		   IBM DB2

       GenericSubQ
	   When all else fails, this should work for many databases, but it is probably fairly slow.

	   This method relies on having a column with unique values as the first column in the "SELECT" clause (i.e. the first column in the
	   "@fields" parameter). The results will be sorted by that unique column, so any $order parameter is ignored, unless it matches the
	   unique column, in which case the direction of the sort is honoured.

	   Syntax
		       SELECT field_list FROM $table X WHERE where_clause AND
		       (
			   SELECT COUNT(*) FROM $table WHERE $pk > X.$pk
		       )
		       BETWEEN $offset AND $last
		       ORDER BY $pk $asc_desc

		   $pk is the first column in "field_list".

		   $asc_desc is the opposite direction to that specified in the method call. So if you want the final results sorted "ASC", say
		   so, and it gets flipped internally, but the results come out as you'd expect. I think.

		   The "BETWEEN $offset AND $last" clause is replaced with "< $rows" if <$offset == 0>.

	   Databases
		   Sybase Anything not otherwise known to this module.

       Skip
	   Syntax
		     select skip 5 limit 5 * from customer

		   which will take rows 6 through 10 in the select.

	   Databases
		   Informix

SUBCLASSING

       You can create your own syntax by making a subclass that provides an "emulate_limit" method. This might be useful if you are using stored
       procedures to provide more efficient paging.

       emulate_limit( $self, $sql, $order, $rows, $offset )
	   $sql
	       This is the SQL statement built by SQL::Abstract, but without the ORDER BY clause, e.g.

		   SELECT foo, bar FROM my_table WHERE conditions

	       or just

		   WHERE conditions

	       if calling "where" instead of "select".

	   $order
	       The "order" parameter passed to the "select" or "where" call. You can get an "ORDER BY" clause from this by calling

		   my $order_by = $self->_order_by( $order );

	       You can get a pair of "ORDER BY" clauses that sort in opposite directions by saying

		   my ( $up, $down ) = $self->_order_directions( $order );

	   The method should return a suitably modified SQL statement.

AUTO-DETECTING THE DIALECT
       The $dialect parameter that can be passed to the constructor or to the "select" and "where" methods can be a number of things. The module
       will attempt to determine the appropriate syntax to use.

       Supported $dialect things are:

	   dialect name (e.g. LimitOffset, RowsTo, Top etc.)
	   database moniker (e.g. Oracle, SQLite etc.)
	   DBI database handle
	   Class::DBI subclass or object

CAVEATS

       Paging results sets is a complicated undertaking, with several competing factors to take into account. This module does not magically give
       you the optimum paging solution for your situation. It gives you a solution that may be good enough in many situations. But if your tables
       are large, the SQL generated here will often not be efficient. Or if your queries involve joins or other complications, you will probably
       need to look elsewhere.

       But if your tables aren't too huge, and your queries straightforward, you can just plug this module in and move on to your next task.

ACKNOWLEDGEMENTS

       Thanks to Aaron Johnson for the Top syntax model (SQL/Server and MS Access).

       Thanks to Emanuele Zeppieri for the IBM DB2 syntax model.

       Thanks to Paul Falbe for the Informix implementation.

TODO

       Find more syntaxes to implement.

       Test the syntaxes against real databases. I only have access to MySQL. Reports of success or failure would be great.

DEPENDENCIES

       SQL::Abstract, DBI::Const::GetInfoType, Carp.

SEE ALSO

       DBIx::SQLEngine, DBIx::SearchBuilder, DBIx::RecordSet.

BUGS

       Please report all bugs via the CPAN Request Tracker at http://rt.cpan.org/NoAuth/Bugs.html?Dist=SQL-Abstract-Limit
       <http://rt.cpan.org/NoAuth/Bugs.html?Dist=SQL-Abstract-Limit>.

COPYRIGHT AND LICENSE

       Copyright 2004 by David Baird.

       This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

AUTHOR

       David Baird, "cpan@riverside-cms.co.uk"

HOW IS IT DONE ELSEWHERE

       A few CPAN modules do this for a few databases, but the most comprehensive seem to be DBIx::SQLEngine, DBIx::SearchBuilder and
       DBIx::RecordSet.

       Have a look in the source code for my notes on how these modules tackle similar problems.

perl v5.12.4							    2011-09-17						 SQL::Abstract::Limit(3pm)