Monks,

My brain is tired from staring at this. I need your sharp eyes to tell me what I am doing wrong. I am working on the basis of a tutorial on relational modeling of trees and graphs (see http://www.oreillynet.com/lpt/a/2958), aka "preorder traversal." I have the following data

table:
child_node_id  parent_node_id  left_id  right_id
-------------  --------------  -------  --------
2              1             
3              1             
4              1             
5              1             
6              2             
7              2             
8              4             
9              5             
10             7             
9              10   

my $children = $dbh->prepare(qq{
  SELECT child_node_id FROM table WHERE parent_node_id = ?
});

my $setleft = $dbh->prepare(qq{
  UPDATE table SET left_id = ? WHERE child_node_id = ?
});

my $setright = $dbh->prepare(qq{
  UPDATE table SET right_id = ? WHERE child_node_id = ?
});

my $ctr = 1;
my $rootid = 1;

walktree($rootid);

sub walktree {
  my ($id) = @_;
  
  $setleft->execute($ctr++, $id);
  $children->execute($id);
  
  while(my ($subid) = $children->fetchrow_array) {
    walktree($subid);
  }
  
  $setright->execute($ctr++, $id);
}
[download]

However, the code shown above does not work. It starts off with

• parent_node_id = 1 and finds its four child_nodes (2, 3, 4, and 5)
  • starts working on parent_node_id = 2 (the first of the four nodes found above) and finds its two child_nodes (6 and 7)
    • starts working on parent_node_id = 6 (the first of the two nodes found above)
    • since 6 is a terminal node (no child nodes for 6), it is supposed to go on to 7, but that doesn't happen

What am I doing wrong?

Update: Solved. See replies from gamache and blokhead below.

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