Add a categoryParent field to the category table. Set it to zero for main level categories, and set the categoryID to subCategories.... then you need to ask yourself do all subcategories exist in only one category?
That would of course also effect your database structure... but you don't want to make things so complex that you go over your head, so consider carefully.
If you did want subcategories to exist within different subcategories, or products to exist within different subcategories.. these decisions would go into how you would structure your database.
For instance if products could exist in more than one category, then you wouldn't have a categoryID as part of the product, instead you would have a productCategories table that included the categoryID and the productID so that one product could be listed in more than one category.
Similarly if you had more than one parent category for a subcategory, then you would not add the parentID to the category table and instead you would create a parentCategory table with the parentCategoryID and childCategoryID so that those too could exist in more than one structure. But of course these things will all complicate the creation of the administrative section to update the necessary information properly and easily.
