{
    my $sponge = DBI->connect(
        'dbi:Sponge:', '', '',
        { RaiseError => 1 }
    );

    my $sth = $sponge->prepare(
        'SELECT id, parent_id FROM Table',
        {
            NAME => [qw( id parent_id )],
            rows => [
                [ a => undef ],
                [ b => 'a'   ],
                [ d => 'c'   ],
                [ c => 'b'   ],
                [ e => 'a'   ],
            ],
        }
    );

    my $tree = treeify($sth);

    use Data::Dumper;
    print Dumper $tree;
}