Can anyone suggest a way to make this work?

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
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If you're stuck with an older version of MySQL, you're probably faced with running the two queries separately, then merging the results.