So I have a Categories table with top level categories and sub categories that have a parent_id column to associate the sub category to the top level.

I then have an Items table with a category_id that associates the item to the category. The category_id always points to a sub category.

How would I get all the items that are under a top level category?

As an example, say I have the following category setup:

Photoshop
--Text Effects
--Basics

All of the items point to the category id's of those sub categories, but I'm wanting to get all the items under "Photoshop".