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Filename : Cookbook.pod

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=head1 NAME

DBD::SQLite::Cookbook - The DBD::SQLite Cookbook

=head1 DESCRIPTION

This is the L<DBD::SQLite> cookbook.

It is intended to provide a place to keep a variety of functions and
formals for use in callback APIs in L<DBD::SQLite>.

=head1 AGGREGATE FUNCTIONS

=head2 Variance

This 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;

=head2 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;

=head2 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;


=head1 SUPPORT

Bugs should be reported via the CPAN bug tracker at

L<http://rt.cpan.org/NoAuth/ReportBug.html?Queue=DBD-SQLite>

=head1 TO DO

=over

=item * 

Add more and varied cookbook recipes, until we have enough to
turn them into a separate CPAN distribution.

=item * 

Create a series of tests scripts that validate the cookbook recipes.

=back

=head1 AUTHOR

Adam Kennedy E<lt>adamk@cpan.orgE<gt>

=head1 COPYRIGHT

Copyright 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.

Name	Size	Last Modified	Owner	Permissions
VirtualTable	Dir	July 28 2023 2:57:47	root	0755
Constants.pm	16.176 KB	November 20 2017 5:52:02	root	0644
Cookbook.pod	3.662 KB	April 21 2016 1:44:34	root	0644
Fulltext_search.pod	18.732 KB	April 21 2016 1:44:34	root	0644
VirtualTable.pm	21.837 KB	March 28 2018 11:52:17	root	0644

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