1DBD::SQLite::Cookbook(3U)ser Contributed Perl DocumentatiDoBnD::SQLite::Cookbook(3)
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NAME

6       DBD::SQLite::Cookbook - The DBD::SQLite Cookbook
7

DESCRIPTION

9       This is the DBD::SQLite cookbook.
10
11       It is intended to provide a place to keep a variety of functions and
12       formals for use in callback APIs in DBD::SQLite.
13

AGGREGATE FUNCTIONS

15   Variance
16       This is a simple aggregate function which returns a variance. It is
17       adapted from an example implementation in pysqlite.
18
19         package variance;
20
21         sub new { bless [], shift; }
22
23         sub step {
24             my ( $self, $value ) = @_;
25
26             push @$self, $value;
27         }
28
29         sub finalize {
30             my $self = $_[0];
31
32             my $n = @$self;
33
34             # Variance is NULL unless there is more than one row
35             return undef unless $n || $n == 1;
36
37             my $mu = 0;
38             foreach my $v ( @$self ) {
39                 $mu += $v;
40             }
41             $mu /= $n;
42
43             my $sigma = 0;
44             foreach my $v ( @$self ) {
45                 $sigma += ($v - $mu)**2;
46             }
47             $sigma = $sigma / ($n - 1);
48
49             return $sigma;
50         }
51
52         # NOTE: If you use an older DBI (< 1.608),
53         # use $dbh->func(..., "create_aggregate") instead.
54         $dbh->sqlite_create_aggregate( "variance", 1, 'variance' );
55
56       The function can then be used as:
57
58         SELECT group_name, variance(score)
59         FROM results
60         GROUP BY group_name;
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62   Variance (Memory Efficient)
63       A more efficient variance function, optimized for memory usage at the
64       expense of precision:
65
66         package variance2;
67
68         sub new { bless {sum => 0, count=>0, hash=> {} }, shift; }
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70         sub step {
71             my ( $self, $value ) = @_;
72             my $hash = $self->{hash};
73
74             # by truncating and hashing, we can comsume many more data points
75             $value = int($value); # change depending on need for precision
76                                   # use sprintf for arbitrary fp precision
77             if (exists $hash->{$value}) {
78                 $hash->{$value}++;
79             } else {
80                 $hash->{$value} = 1;
81             }
82             $self->{sum} += $value;
83             $self->{count}++;
84         }
85
86         sub finalize {
87             my $self = $_[0];
88
89             # Variance is NULL unless there is more than one row
90             return undef unless $self->{count} > 1;
91
92             # calculate avg
93             my $mu = $self->{sum} / $self->{count};
94
95             my $sigma = 0;
96             while (my ($h, $v) = each %{$self->{hash}}) {
97                 $sigma += (($h - $mu)**2) * $v;
98             }
99             $sigma = $sigma / ($self->{count} - 1);
100
101             return $sigma;
102         }
103
104       The function can then be used as:
105
106         SELECT group_name, variance2(score)
107         FROM results
108         GROUP BY group_name;
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110   Variance (Highly Scalable)
111       A third variable implementation, designed for arbitrarily large data
112       sets:
113
114         package variance3;
115
116         sub new { bless {mu=>0, count=>0, S=>0}, shift; }
117
118         sub step {
119             my ( $self, $value ) = @_;
120             $self->{count}++;
121             my $delta = $value - $self->{mu};
122             $self->{mu} += $delta/$self->{count};
123             $self->{S} += $delta*($value - $self->{mu});
124         }
125
126         sub finalize {
127             my $self = $_[0];
128             return $self->{S} / ($self->{count} - 1);
129         }
130
131       The function can then be used as:
132
133         SELECT group_name, variance3(score)
134         FROM results
135         GROUP BY group_name;
136

SUPPORT

138       Bugs should be reported via the CPAN bug tracker at
139
140       <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=DBD-SQLite>
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TO DO

143       ·   Add more and varied cookbook recipes, until we have enough to turn
144           them into a separate CPAN distribution.
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146       ·   Create a series of tests scripts that validate the cookbook
147           recipes.
148

AUTHOR

150       Adam Kennedy <adamk@cpan.org>
151

153       Copyright 2009 - 2012 Adam Kennedy.
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155       This program is free software; you can redistribute it and/or modify it
156       under the same terms as Perl itself.
157
158       The full text of the license can be found in the LICENSE file included
159       with this module.
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163perl v5.30.1                      2020-02-04          DBD::SQLite::Cookbook(3)
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