$sth = $dbh->prepare(q{
  SELECT id, name
  FROM category
  WHERE parentCategoryId = ?
  ORDER BY name
});
my @sub_categories;
my @included_categories = ($root_category_id);
my $included_categories_index = 0;
while ($included_categories_index <= $#included_categories) {
  $sth->execute($included_categories[$included_categories_index]);
  while ($_ = $sth->fetchrow_hashref) {
    push @sub_categories, $_
      if ($included_categories[$included_categories_index] == $root_category_id);
    push @included_categories, $_->{id};
  }
  $included_categories_index++;
}