Modified preorder tree traversal: Selecting nodes 1 level deep

Question

I have hierarchical ordered data saved using the modified preorder tree traversal algorithm.

Here's tables content:

id  lft  rgt  name
1   1    10   topnode
2   2    3    level1
3   4    7    level1
4   5    6    level2
5   8    9    level1

Visualised:

What I want is to select just the childnodes of a certain node (so not the childnodes of the childnodes). Let's say 'topnode'. I'm trying to fix a query, but I can't seem to get my head around it.

Searching the internet brings me a while, for example: I can calculate the depth of each node, but I just can't seem to select on it.

This query

SELECT node.*, (COUNT(parent.id) - 1) AS depth
FROM tree AS node
CROSS JOIN tree AS parent
WHERE (node.lft BETWEEN parent.lft AND parent.rgt)
GROUP BY node.id
ORDER BY node.lft

shows the depth of each node:

id  lft  rgt  name     depth
1   1    10   topnode  0
2   2    3    level1   1
3   4    7    level1   1
4   5    6    level2   2
5   8    9    level1   1

That's great, but I can't use the column depth as a condition!

Answer 1

I think this should do the trick

SELECT node.*, (COUNT(parent.id) - 1) AS depth
FROM tree AS node
CROSS JOIN tree AS parent
WHERE (node.lft BETWEEN parent.lft AND parent.rgt)
GROUP BY node.id
HAVING depth = {{ENTER YOUR REQUIRED DEPTH HERE}}
ORDER BY node.lft

Hope that helps