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.\" ========================================================================
.\"
.IX Title "SQL::Abstract::Limit 3pm"
.TH SQL::Abstract::Limit 3pm "2011-09-17" "perl v5.12.4" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
SQL::Abstract::Limit \- portable LIMIT emulation
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&    use SQL::Abstract::Limit;
\&
\&    my $sql = SQL::Abstract::Limit\->new( limit_dialect => \*(AqLimitOffset\*(Aq );;
\&
\&    # 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 => \*(AqMy::CDBI::App\*(Aq );
\&
\&    # 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, \e@fields, \e%where, \e@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( \e%where, \e@order, $limit, $offset );
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
Portability layer for \s-1LIMIT\s0 emulation.
.IP "new( case => 'lower', cmp => 'like', logic => 'and', convert => 'upper', limit_dialect => 'Top' )" 4
.IX Item "new( case => 'lower', cmp => 'like', logic => 'and', convert => 'upper', limit_dialect => 'Top' )"
All settings are optional.
.RS 4
.IP "limit_dialect" 8
.IX Item "limit_dialect"
Sets the default syntax model to use for emulating a \f(CW\*(C`LIMIT $rows OFFSET $offset\*(C'\fR
clause. Default setting is \f(CW\*(C`GenericSubQ\*(C'\fR. You can still pass other syntax
settings in method calls, this just sets the default. Possible values are:
.Sp
.Vb 4
\&    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
\&
\&    $dbh            a DBI database handle
\&
\&    CDBI subclass
\&    CDBI object
\&
\&    other DBI\-based thing
.Ve
.Sp
The first group are implemented by appending a short clause to the end of the
statement. The second group require more intricate wrapping of the original
statement in subselects.
.Sp
You can pass a \s-1DBI\s0 database handle, and the module will figure out which
dialect to use.
.Sp
You can pass a Class::DBI subclass or object, and the module will
find the \f(CW$dbh\fR and use it to find the dialect.
.Sp
Anything else based on \s-1DBI\s0 can be easily added by locating the \f(CW$dbh\fR.
Patches or suggestions welcome.
.RE
.RS 4
.Sp
Other options are described in SQL::Abstract.
.RE
.ie n .IP "select( $table, \e@fields, $where, [ \e@order, [ $rows, [ $offset ], [ $dialect ] ] ] )" 4
.el .IP "select( \f(CW$table\fR, \e@fields, \f(CW$where\fR, [ \e@order, [ \f(CW$rows\fR, [ \f(CW$offset\fR ], [ \f(CW$dialect\fR ] ] ] )" 4
.IX Item "select( $table, @fields, $where, [ @order, [ $rows, [ $offset ], [ $dialect ] ] ] )"
Same as \f(CW\*(C`SQL::Abstract::select\*(C'\fR, but accepts additional \f(CW$rows\fR, \f(CW$offset\fR
and \f(CW$dialect\fR parameters.
.Sp
The \f(CW$order\fR parameter is required if \f(CW$rows\fR is specified.
.Sp
The \f(CW$fields\fR parameter is required, but can be set to \f(CW\*(C`undef\*(C'\fR, \f(CW\*(Aq\*(Aq\fR or
\&\f(CW\*(Aq*\*(Aq\fR (all these get set to \f(CW\*(Aq*\*(Aq\fR).
.Sp
The \f(CW$where\fR parameter is also required. It can be a hashref 
or an arrayref, or \f(CW\*(C`undef\*(C'\fR.
.ie n .IP "where( [ $where, [ \e@order, [ $rows, [ $offset ], [ $dialect ] ] ] ] )" 4
.el .IP "where( [ \f(CW$where\fR, [ \e@order, [ \f(CW$rows\fR, [ \f(CW$offset\fR ], [ \f(CW$dialect\fR ] ] ] ] )" 4
.IX Item "where( [ $where, [ @order, [ $rows, [ $offset ], [ $dialect ] ] ] ] )"
Same as \f(CW\*(C`SQL::Abstract::where\*(C'\fR, but accepts additional \f(CW$rows\fR, \f(CW$offset\fR
and \f(CW$dialect\fR parameters.
.Sp
Some \s-1SQL\s0 dialects support syntaxes that can be applied as simple phrases
tacked on to the end of the \s-1WHERE\s0 clause. These are:
.Sp
.Vb 4
\&    LimitOffset
\&    LimitXY
\&    LimitYX
\&    RowsTo
.Ve
.Sp
This method returns a modified \s-1WHERE\s0 clause, if the limit syntax is set to one
of these options (either in the call to \f(CW\*(C`where\*(C'\fR or in the constructor), and
if \f(CW$rows\fR is passed in.
.Sp
Dies via \f(CW\*(C`croak\*(C'\fR if you try to use it for other syntaxes.
.Sp
\&\f(CW$order\fR is required if \f(CW$rows\fR is set.
.Sp
\&\f(CW$where\fR is required if any other parameters are specified. It can be a hashref 
or an arrayref, or \f(CW\*(C`undef\*(C'\fR.
.Sp
Returns a regular \f(CW\*(C`WHERE\*(C'\fR clause if no limits are set.
.IP "insert" 4
.IX Item "insert"
.PD 0
.IP "update" 4
.IX Item "update"
.IP "delete" 4
.IX Item "delete"
.IP "values" 4
.IX Item "values"
.IP "generate" 4
.IX Item "generate"
.PD
See SQL::Abstract for these methods.
.Sp
\&\f(CW\*(C`update\*(C'\fR and \f(CW\*(C`delete\*(C'\fR are not provided with any \f(CW\*(C`LIMIT\*(C'\fR emulation in this
release, and no support is planned at the moment. But patches would be welcome.
.SS "Limit emulation"
.IX Subsection "Limit emulation"
The following dialects are available for emulating the \s-1LIMIT\s0 clause. In each
case, \f(CW$sql\fR represents the \s-1SQL\s0 statement generated by \f(CW\*(C`SQL::Abstract::select\*(C'\fR,
minus the \s-1ORDER\s0 \s-1BY\s0 clause, e.g.
.PP
.Vb 1
\&    SELECT foo, bar FROM my_table WHERE some_conditions
.Ve
.PP
\&\f(CW$sql_after_select\fR represents \f(CW$sql\fR with the leading \f(CW\*(C`SELECT\*(C'\fR keyword
removed.
.PP
\&\f(CW\*(C`order_cols_up\*(C'\fR represents the sort column(s) and direction(s) specified in
the \f(CW\*(C`order\*(C'\fR parameter.
.PP
\&\f(CW\*(C`order_cols_down\*(C'\fR represents the opposite sort.
.PP
\&\f(CW\*(C`$last = $rows + $offset\*(C'\fR
.IP "LimitOffset" 4
.IX Item "LimitOffset"
.RS 4
.PD 0
.IP "Syntax" 8
.IX Item "Syntax"
.PD
.Vb 1
\&    $sql ORDER BY order_cols_up LIMIT $rows OFFSET $offset
.Ve
.Sp
or
.Sp
.Vb 1
\&    $sql ORDER BY order_cols_up LIMIT $rows
.Ve
.Sp
if \f(CW\*(C`$offset == 0\*(C'\fR.
.IP "Databases" 8
.IX Item "Databases"
.Vb 2
\&    PostgreSQL
\&    SQLite
.Ve
.RE
.RS 4
.RE
.IP "LimitXY" 4
.IX Item "LimitXY"
.RS 4
.PD 0
.IP "Syntax" 8
.IX Item "Syntax"
.PD
.Vb 1
\&    $sql ORDER BY order_cols_up LIMIT $offset, $rows
.Ve
.Sp
or
.Sp
.Vb 1
\&     $sql ORDER BY order_cols_up LIMIT $rows
.Ve
.Sp
if \f(CW\*(C`$offset == 0\*(C'\fR.
.IP "Databases" 8
.IX Item "Databases"
.Vb 1
\&    MySQL
.Ve
.RE
.RS 4
.RE
.IP "LimitYX" 4
.IX Item "LimitYX"
.RS 4
.PD 0
.IP "Syntax" 8
.IX Item "Syntax"
.PD
.Vb 1
\&    $sql ORDER BY order_cols_up LIMIT $rows, $offset
.Ve
.Sp
or
.Sp
.Vb 1
\&    $sql ORDER BY order_cols_up LIMIT $rows
.Ve
.Sp
if \f(CW\*(C`$offset == 0\*(C'\fR.
.IP "Databases" 8
.IX Item "Databases"
.Vb 2
\&    SQLite understands this syntax, or LimitOffset. If autodetecting the
\&           dialect, it will be set to LimitOffset.
.Ve
.RE
.RS 4
.RE
.IP "RowsTo" 4
.IX Item "RowsTo"
.RS 4
.PD 0
.IP "Syntax" 8
.IX Item "Syntax"
.PD
.Vb 1
\&    $sql ORDER BY order_cols_up ROWS $offset TO $last
.Ve
.IP "Databases" 8
.IX Item "Databases"
.Vb 2
\&    InterBase
\&    FireBird
.Ve
.RE
.RS 4
.RE
.IP "Top" 4
.IX Item "Top"
.RS 4
.PD 0
.IP "Syntax" 8
.IX Item "Syntax"
.PD
.Vb 10
\&    SELECT * FROM
\&    (
\&        SELECT TOP $rows * FROM
\&        (
\&            SELECT TOP $last $sql_after_select
\&            ORDER BY order_cols_up
\&        ) AS foo
\&        ORDER BY order_cols_down
\&    ) AS bar
\&    ORDER BY order_cols_up
.Ve
.IP "Databases" 8
.IX Item "Databases"
.Vb 2
\&    SQL/Server
\&    MS Access
.Ve
.RE
.RS 4
.RE
.IP "RowNum" 4
.IX Item "RowNum"
.RS 4
.PD 0
.IP "Syntax" 8
.IX Item "Syntax"
.PD
Oracle numbers rows from 1, not zero, so here \f(CW$offset\fR has been incremented by 1.
.Sp
.Vb 9
\&    SELECT * FROM
\&    (
\&        SELECT A.*, ROWNUM r FROM
\&        (
\&            $sql ORDER BY order_cols_up
\&        ) A
\&        WHERE ROWNUM <= $last
\&    ) B
\&    WHERE r >= $offset
.Ve
.IP "Databases" 8
.IX Item "Databases"
.Vb 1
\&    Oracle
.Ve
.RE
.RS 4
.RE
.IP "FetchFirst" 4
.IX Item "FetchFirst"
.RS 4
.PD 0
.IP "Syntax" 8
.IX Item "Syntax"
.PD
.Vb 10
\&    SELECT * FROM (
\&        SELECT * FROM (
\&            $sql
\&            ORDER BY order_cols_up
\&            FETCH FIRST $last ROWS ONLY
\&        ) foo
\&        ORDER BY order_cols_down
\&        FETCH FIRST $rows ROWS ONLY
\&    ) bar
\&    ORDER BY order_cols_up
.Ve
.IP "Databases" 8
.IX Item "Databases"
\&\s-1IBM\s0 \s-1DB2\s0
.RE
.RS 4
.RE
.IP "GenericSubQ" 4
.IX Item "GenericSubQ"
When all else fails, this should work for many databases, but it is probably
fairly slow.
.Sp
This method relies on having a column with unique values as the first column in
the \f(CW\*(C`SELECT\*(C'\fR clause (i.e. the first column in the \f(CW\*(C`\e@fields\*(C'\fR parameter). The
results will be sorted by that unique column, so any \f(CW$order\fR parameter is
ignored, unless it matches the unique column, in which case the direction of
the sort is honoured.
.RS 4
.IP "Syntax" 8
.IX Item "Syntax"
.Vb 6
\&    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 $asc_desc
.Ve
.Sp
\&\f(CW$pk\fR is the first column in \f(CW\*(C`field_list\*(C'\fR.
.Sp
\&\f(CW$asc_desc\fR is the opposite direction to that specified in the method call. So
if you want the final results sorted \f(CW\*(C`ASC\*(C'\fR, say so, and it gets flipped
internally, but the results come out as you'd expect. I think.
.Sp
The \f(CW\*(C`BETWEEN $offset AND $last\*(C'\fR clause is replaced with \f(CW\*(C`< $rows\*(C'\fR if
<$offset == 0>.
.IP "Databases" 8
.IX Item "Databases"
Sybase
Anything not otherwise known to this module.
.RE
.RS 4
.RE
.IP "Skip" 4
.IX Item "Skip"
.RS 4
.PD 0
.IP "Syntax" 8
.IX Item "Syntax"
.PD
.Vb 1
\&  select skip 5 limit 5 * from customer
.Ve
.Sp
which will take rows 6 through 10 in the select.
.IP "Databases" 8
.IX Item "Databases"
Informix
.RE
.RS 4
.RE
.SH "SUBCLASSING"
.IX Header "SUBCLASSING"
You can create your own syntax by making a subclass that provides an
\&\f(CW\*(C`emulate_limit\*(C'\fR method. This might be useful if you are using stored procedures
to provide more efficient paging.
.ie n .IP "emulate_limit( $self, $sql, $order, $rows, $offset )" 4
.el .IP "emulate_limit( \f(CW$self\fR, \f(CW$sql\fR, \f(CW$order\fR, \f(CW$rows\fR, \f(CW$offset\fR )" 4
.IX Item "emulate_limit( $self, $sql, $order, $rows, $offset )"
.RS 4
.PD 0
.ie n .IP "$sql" 4
.el .IP "\f(CW$sql\fR" 4
.IX Item "$sql"
.PD
This is the \s-1SQL\s0 statement built by SQL::Abstract, but without
the \s-1ORDER\s0 \s-1BY\s0 clause, e.g.
.Sp
.Vb 1
\&    SELECT foo, bar FROM my_table WHERE conditions
.Ve
.Sp
or just
.Sp
.Vb 1
\&    WHERE conditions
.Ve
.Sp
if calling \f(CW\*(C`where\*(C'\fR instead of \f(CW\*(C`select\*(C'\fR.
.ie n .IP "$order" 4
.el .IP "\f(CW$order\fR" 4
.IX Item "$order"
The \f(CW\*(C`order\*(C'\fR parameter passed to the \f(CW\*(C`select\*(C'\fR or \f(CW\*(C`where\*(C'\fR call. You can get
an \f(CW\*(C`ORDER BY\*(C'\fR clause from this by calling
.Sp
.Vb 1
\&    my $order_by = $self\->_order_by( $order );
.Ve
.Sp
You can get a pair of \f(CW\*(C`ORDER BY\*(C'\fR clauses that sort in opposite directions by
saying
.Sp
.Vb 1
\&    my ( $up, $down ) = $self\->_order_directions( $order );
.Ve
.RE
.RS 4
.Sp
The method should return a suitably modified \s-1SQL\s0 statement.
.RE
.SH "AUTO-DETECTING THE DIALECT"
.IX Header "AUTO-DETECTING THE DIALECT"
The \f(CW$dialect\fR parameter that can be passed to the constructor or to the
\&\f(CW\*(C`select\*(C'\fR and \f(CW\*(C`where\*(C'\fR methods can be a number of things. The module will
attempt to determine the appropriate syntax to use.
.PP
Supported \f(CW$dialect\fR things are:
.PP
.Vb 4
\&    dialect name (e.g. LimitOffset, RowsTo, Top etc.)
\&    database moniker (e.g. Oracle, SQLite etc.)
\&    DBI database handle
\&    Class::DBI subclass or object
.Ve
.SH "CAVEATS"
.IX Header "CAVEATS"
Paging results sets is a complicated undertaking, with several competing factors
to take into account. This module does \fBnot\fR 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 \s-1SQL\s0 generated here
will often not be efficient. Or if your queries involve joins or other
complications, you will probably need to look elsewhere.
.PP
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.
.SH "ACKNOWLEDGEMENTS"
.IX Header "ACKNOWLEDGEMENTS"
Thanks to Aaron Johnson for the Top syntax model (SQL/Server and \s-1MS\s0 Access).
.PP
Thanks to Emanuele Zeppieri for the \s-1IBM\s0 \s-1DB2\s0 syntax model.
.PP
Thanks to Paul Falbe for the Informix implementation.
.SH "TODO"
.IX Header "TODO"
Find more syntaxes to implement.
.PP
Test the syntaxes against real databases. I only have access to MySQL. Reports
of success or failure would be great.
.SH "DEPENDENCIES"
.IX Header "DEPENDENCIES"
SQL::Abstract,
DBI::Const::GetInfoType,
Carp.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
DBIx::SQLEngine,
DBIx::SearchBuilder,
DBIx::RecordSet.
.SH "BUGS"
.IX Header "BUGS"
Please report all bugs via the \s-1CPAN\s0 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>.
.SH "COPYRIGHT AND LICENSE"
.IX Header "COPYRIGHT AND LICENSE"
Copyright 2004 by David Baird.
.PP
This library is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.
.SH "AUTHOR"
.IX Header "AUTHOR"
David Baird, \f(CW\*(C`cpan@riverside\-cms.co.uk\*(C'\fR
.SH "HOW IS IT DONE ELSEWHERE"
.IX Header "HOW IS IT DONE ELSEWHERE"
A few \s-1CPAN\s0 modules do this for a few databases, but the most comprehensive
seem to be DBIx::SQLEngine, DBIx::SearchBuilder and DBIx::RecordSet.
.PP
Have a look in the source code for my notes on how these modules tackle
similar problems.