sub get_category_loop {

    return [
        map { 
            $_->{subject_loop} 
                = get_subject_loop( delete $_->{category_id} ); # delete returns the value that was in the hash
            $_
        }
        @{ $dbh->selectall_arrayref(sql_category(), {Slice => {}}) }
    ];

}

sub get_subject_loop {

    my $category_id = shift;

    my $sth = $dbh->prepare_cached( sql_subject() );
    $sth->execute($category_id);
    return $sth->fetchall_arrayref({});

}

sub sql_category {

    return q{
        SELECT   category_id,
                 category_name
        FROM     category
        ORDER BY category_id
    };

}

sub sql_subject {

    return q{
        SELECT   subject_id,
                 subject_name
        FROM     subject
        WHERE    category_id = ?
        ORDER BY subject_name
    };

}