Question

I know there’s an easy way of doing this, but my recursion abilities are out of practice. Given a database table that has three fields:

id
label
child_id

I should be able to put together a recursive function that will give output like this:

child (input of program)
  parent1
  parent2
    grandparent1
      great-grandparent1
    grandparent2
    grandparent3
  parent3
    grandparent4
    grandparent5

I know it should be easy, but I can’t get my mind to go through the mental gymnastics to make it work. Also, is this a good thing to do? Seems like I might end up leaving open quite a few database connections.

I think this is the part making it difficult for me. I’m starting with a child_id, and working my way up. And a child can have many parents. So, the output would be the child id at the ‘root’ of the tree and then it’s parents and grandparents for each branch. The more I think about it, it’s just the traditional ‘one parent, many grandparents’ formula, except for semantics. I could just be over thinking it.

The table would look something like this:

table parents

id    child_id    label
 1     NULL       child
 2     1          parent1
 3     1          parent2
 4     1          parent3
 5     3          grandparent1
 6     3          grandparent2
 7     3          grandparent3
 8     5          great-grandparent1
 9     4          grandparent4
10     4          grandparent5

Answer 1

You could try this way

sub getChildren {
  my $id = shift;
  my $depth = shift;
  my $sql = qq/SELECT id,label,child_id FROM table WHERE id=?/;
  my $sth = $db->prepare($sql);
  my $sth->execute($id);
  while(my ($id,$label,$child_id)=$sth->fetchrow_array) {
    print " "x$depth,$label;
    getChildren($child_id,$depth++);
 }
}
getChildren($id);