SQL nested order by?

Question

I\'m sure that this has been asked before, but I don\'t know what to call it exactly to find the answer.

I have a table of categories and sub categories. They each

Answer 1

Something like this might maybe work:

SELECT *
FROM categories
ORDER BY IF(parent_id, parent_id, category_id), parent_id, display_order

but since it can't use an index, it'll be slow. (Didn't test though, might be wrong)

The first ORDER BY condition sorts parents and children together; then the second one ensures the parent precedes its children; the third sorts the children among themselves.

Also, it will obviously work only in the case you directly described, where you have a two-level hierarchy.