complex mysql query in mysql 3.23

[travis]

Hi all, here's my situation

I have hierarchical data that can be an arbitrary number of levels deep. I'm representing it with the adjacency list model. So there are 2 tables

Categories
id, parentid, title

and

DOCUMENTS
id, parentid, title, desc


Now, here is what I am already doing successfully - I can retrieve a resultset which contains mixed results from both of these tables - essentially a union statement, though because i am forced to use mysql 3.23 i am using the dummy table method. The query looks like this...

SELECT
IFNULL(documents.id, categories.id) AS id,
IFNULL(documents.parentid, categories.parentid) AS parentid,
IFNULL(documents.title, categories.title) AS title,
documents.desc,
IF(categories.id, 1, 0) AS isCategory
FROM _dummy AS D
LEFT JOIN documents ON (D.num = 0 AND documents.parentid = $parentID)
LEFT JOIN categories ON (D.num = 1 AND categories.parentid = $parentID)
WHERE D.num < 2 AND IFNULL(documents.id, categories.id) IS NOT NULL


This works like a charm. However, if possible I would also like to retrieve an additional field which is a COUNT() of the immediate children (documents and categories only one level deep) of each 'category' in the resultset. Mysql 3.23 doesn't support subselects, but I'm fairly certain this is possible with temporary tables. However, I'm not entirely sure how to go about it. (Have been beating my head against this for awhile, grin)

If anyone has some suggestions, I'd be deeply appreciative. Currently I'm just looping through the resultset and executing a separate COUNT() on categories - which I'd prefer to avoid if possible.

Thanks in advance for your help.