Finding all non-empty categories in a tree

[Obbamahaa]

I have a categories table set up as so [id, name, parent_id] and a items table [id, name, category_id, visible]. What I'm trying to do is create a query that will return all the ids of all non-empty categories, non empty being that it or one of it's children has at least one item belonging to it. What would be the best way to do this in MySQL?editSELECT DISTINCT category_id FROM ItemsThis works for categories containing items, but I also need the parent categories of all item containing categories. This query will be used as a subquery along with some other filters.Top Level Category->Second Level Category-->Third Level Category--->Item 1--->Item 2