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.\" ========================================================================
.\"
.IX Title "DBD::SQLite::Cookbook 3"
.TH DBD::SQLite::Cookbook 3 "2024-01-21" "perl v5.30.0" "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
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.SH "NAME"
DBD::SQLite::Cookbook \- The DBD::SQLite Cookbook
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
This is the DBD::SQLite cookbook.
.PP
It is intended to provide a place to keep a variety of functions and
formals for use in callback APIs in DBD::SQLite.
.SH "AGGREGATE FUNCTIONS"
.IX Header "AGGREGATE FUNCTIONS"
.SS "Variance"
.IX Subsection "Variance"
This is a simple aggregate function which returns a variance. It is
adapted from an example implementation in pysqlite.
.PP
.Vb 1
\& 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, \*(Aqvariance\*(Aq );
.Ve
.PP
The function can then be used as:
.PP
.Vb 3
\& SELECT group_name, variance(score)
\& FROM results
\& GROUP BY group_name;
.Ve
.SS "Variance (Memory Efficient)"
.IX Subsection "Variance (Memory Efficient)"
A more efficient variance function, optimized for memory usage at the
expense of precision:
.PP
.Vb 1
\& 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;
\& }
.Ve
.PP
The function can then be used as:
.PP
.Vb 3
\& SELECT group_name, variance2(score)
\& FROM results
\& GROUP BY group_name;
.Ve
.SS "Variance (Highly Scalable)"
.IX Subsection "Variance (Highly Scalable)"
A third variable implementation, designed for arbitrarily large data sets:
.PP
.Vb 1
\& 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);
\& }
.Ve
.PP
The function can then be used as:
.PP
.Vb 3
\& SELECT group_name, variance3(score)
\& FROM results
\& GROUP BY group_name;
.Ve
.SH "SUPPORT"
.IX Header "SUPPORT"
Bugs should be reported via the \s-1CPAN\s0 bug tracker at
.PP
<http://rt.cpan.org/NoAuth/ReportBug.html?Queue=DBD\-SQLite>
.SH "TO DO"
.IX Header "TO DO"
.IP "\(bu" 4
Add more and varied cookbook recipes, until we have enough to
turn them into a separate \s-1CPAN\s0 distribution.
.IP "\(bu" 4
Create a series of tests scripts that validate the cookbook recipes.
.SH "AUTHOR"
.IX Header "AUTHOR"
Adam Kennedy <adamk@cpan.org>
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2009 \- 2012 Adam Kennedy.
.PP
This program is free software; you can redistribute
it and/or modify it under the same terms as Perl itself.
.PP
The full text of the license can be found in the
\&\s-1LICENSE\s0 file included with this module.