Question

My SQL database has three columns; Key, Topic, and Value. There are duplicate topics that have different values. Here is a sample:

 Key    Topic  Value
 -----  -----  -----
 1      Book      20
 2      Toy       10
 3      Toy       30
 4      Pet      100
 5      Book      15
 .      .          .
 .      .          .

Using Perl, how do I create a hash from the column Topic (%Topic) that uses each unique key in Topic as an array that has the corresponding values for its elements: @Book = 20, 15, ..., @Toy = 10, 30, ..., @Pet = 100, ....

Answer 1

I think you want a hash keyed by Topic, where the value is a reference to an array with the Values for that Topic.

my %values_by_topic;
my $sth = $dbh->prepare('SELECT Topic, Value FROM Table');
$sth->execute();
while (my $row = $sth->fetch()) {
   my ($topic, $value) = @$row;
    push @{ $values_by_topic{$topic} }, $value;
}