This is a common pattern. The basic problem is "how to I generate a child pointer tree from a parent pointer representation (as would normally be stored in a DB)". The following is a (proven but not debugged) solution that doesnt require multiple queries, can handle the data in any order, and only requires a single forward pass over the data.

sub getCategories {
    my ($dbh)=@_;
    
    my $sql = qq~SELECT category_id, parent_id, category_name FROM ss_
+catalog_categories~;
    my $sth = $dbh->prepare($sql);
    $sth->execute;

    my %nodes;
    my @roots;
    my ($cid,$pid,$name);
    while ( ($cid,$pid,$name) = $sth->fetchrow_array()) {
       my $node=($nodes{$cid}||={});
       $node->{name}=$name;
       $node->{pid}=$pid;
       $node->{cid}=$cid;
       if ($pid) {
         $nodes{$pid}{_kids}{$cid}=$node;
       } else {
         push @roots,$node;
       }
    }
    $sth->finish;    
    
    return (\@roots,\%nodes);    
}
[download]

Roots contains the root nodes of your forest, and %nodes contains a hash keyed by the category id of the nodes. You can traverse each tree in the forest like this:

sub traverse {
  my ($node,$depth)=@_;
  $depth||=0;
  print " " x $depth,$node->{cid}," ",$node->{name},"\n";
  foreach my $id ( sort {$a <=> $b} keys %{$node->{_kids}||{}} ) {
    traverse($node->{_kids}{$id},$depth+1);
  }
}
my ($r,$n)=getCategories($dbh);
traverse($_) for sort { $a->{cid} <=> $b->{cid} } @$r;
[download]

For those that care the above code is a highly simplified form of what is used here for the Recently Active Threads.

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