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NAMEDBD::SQLite::Cookbook - The DBD::SQLite CookbookDESCRIPTIONThis is the DBD::SQLite cookbook.It is intended to provide a place to keep a variety of functions and formals for use in callback APIs in DBD::SQLite. AGGREGATE FUNCTIONSVarianceThis is a simple aggregate function which returns a variance. It is adapted from an example implementation in pysqlite.package variance; sub new { bless [], shift; } sub step { my ( $self, $value ) = @_; push @$self, $value; } sub finalize { my $self = $_[0]; my $n = @$self; # Variance is NULL unless there is more than one row return undef unless $n || $n == 1; my $mu = 0; foreach my $v ( @$self ) { $mu += $v; } $mu /= $n; my $sigma = 0; foreach my $v ( @$self ) { $sigma += ($v - $mu)**2; } $sigma = $sigma / ($n - 1); return $sigma; } # NOTE: If you use an older DBI (< 1.608), # use $dbh->func(..., "create_aggregate") instead. $dbh->sqlite_create_aggregate( "variance", 1, 'variance' ); The function can then be used as: SELECT group_name, variance(score) FROM results GROUP BY group_name; Variance (Memory Efficient)A more efficient variance function, optimized for memory usage at the expense of precision:package variance2; sub new { bless {sum => 0, count=>0, hash=> {} }, shift; } sub step { my ( $self, $value ) = @_; my $hash = $self->{hash}; # by truncating and hashing, we can comsume many more data points $value = int($value); # change depending on need for precision # use sprintf for arbitrary fp precision if (exists $hash->{$value}) { $hash->{$value}++; } else { $hash->{$value} = 1; } $self->{sum} += $value; $self->{count}++; } sub finalize { my $self = $_[0]; # Variance is NULL unless there is more than one row return undef unless $self->{count} > 1; # calculate avg my $mu = $self->{sum} / $self->{count}; my $sigma = 0; while (my ($h, $v) = each %{$self->{hash}}) { $sigma += (($h - $mu)**2) * $v; } $sigma = $sigma / ($self->{count} - 1); return $sigma; } The function can then be used as: SELECT group_name, variance2(score) FROM results GROUP BY group_name; Variance (Highly Scalable)A third variable implementation, designed for arbitrarily large data sets:package variance3; sub new { bless {mu=>0, count=>0, S=>0}, shift; } sub step { my ( $self, $value ) = @_; $self->{count}++; my $delta = $value - $self->{mu}; $self->{mu} += $delta/$self->{count}; $self->{S} += $delta*($value - $self->{mu}); } sub finalize { my $self = $_[0]; return $self->{S} / ($self->{count} - 1); } The function can then be used as: SELECT group_name, variance3(score) FROM results GROUP BY group_name; SUPPORTBugs should be reported via the CPAN bug tracker at<http://rt.cpan.org/NoAuth/ReportBug.html?Queue=DBD-SQLite> TO DO
AUTHORAdam Kennedy <adamk@cpan.org>COPYRIGHTCopyright 2009 - 2012 Adam Kennedy.This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself. The full text of the license can be found in the LICENSE file included with this module.
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