Recursive categories

[eluser]Rick Jolly[/eluser]
JoostV's method will only account for 1 level of child categories. Manilodisan's link refers to the 2 common methods of dealing with hierarchical data in a database:
1) Adjacency List
2) Modified Preorder Tree Traversal aka Nested Sets.

The adjacency list is simpler, but less efficient for select queries. If your categories aren't nested very deep, then I'd use the adjacency list.