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SQL::Abstract::Limit
portable LIMIT emulation

SQL::Abstract::Limit - portable LIMIT emulation


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

    First           Informix    # not implemented yet

    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.

First
Syntax
Looks to be identical to Top, e.g. SELECT FIRST 10 * FROM table. Can probably be implemented in a very similar way, but not done yet.

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.


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.


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.

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