SQL::Abstract::Limit(3p)

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 honored.

    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 (patches welcome) via GitHub at <https://github.com/asb-capfan/SQL-Abstract-Limit> or via the CPAN Request Tracker at <http://rt.cpan.org/NoAuth/Bugs.html?Dist=SQL-Abstract-Limit>.

AUTHOR, COPYRIGHT AND LICENSE
Copyright 2004-2020 by David Baird. Currently maintained by Alexander Becker.

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

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.