You can use tree of parents ant childs:

CREATE TABLE Categories (CATEGORY_ID int unsigned auto_increment primary key, PARENT_ID int unsigned, TITLE valchar(30));

Then you can select categories by parents.
Top level categories can have PARENT_ID=0 or NULL, etc.

Example of tree:

1, 0, category1
2, 0, category2
3, 1, subcat1
4, 1, subcat2
5, 3, subcat1
6, 3, subcat2
7, 3, subcat3