Depth in MYSQL and Closure Table Trees

Question

How would I go about populating a closure table's depth/length column when inserting a new node to the tree?

The values in ancestor and descendant are IDs from another table that represent pages to be arranged in a tree structure.

Closure Table:

ancestor    descendant     depth
1               1            0
1               2            1
1               3            1 
1               4            1
2               2            0
3               3            0 
4               4            0

This will insert the ancestor and descendants properly but I'm not sure how to populate the depth column Insert Query:

INSERT INTO closure_tree_path (ancestor, descendant)
SELECT ancestor, '{$node_id}' FROM closure_tree_path
WHERE descendant = '{$parent_id}'
UNION ALL SELECT '{$node_id}', '{$node_id}';

What's the best way to go about this? Thanks a bunch!

Answer 1

Add depth+1 to the first SELECT.

INSERT INTO closure_tree_path (ancestor, descendant, depth)
SELECT ancestor, '{$node_id}', depth+1 FROM closure_tree_path
WHERE descendant = '{$parent_id}'
UNION ALL SELECT '{$node_id}', '{$node_id}', 0;