MySQL 4.0 adds support for UNION. With that (and assuming table1 and table2 have the same columns), you could do something like:

 SELECT *
 FROM table1
 WHERE table1.category = 1
 UNION
 SELECT *
 FROM table1 JOIN table2 ON table1.category = table2.parent_id
 WHERE table2.parent_id = 1
[download]