Database Design Problems

Hey, I'll *try* to explain the problem im having...I need to know how to set up some tables.
I have a table called "projects" and each project has "requirements". I want there to be up to 4 levels (categories as I named the table) of requirements. BUT a requirement does not have to have a category, and a category obviously does not have to have a requirement. Also, how do I show the parent relationships?

For example how do I show that if category one, has category two below it, AND has a requirement below it? The Idea's i've had and I don't think are right, is having a "parent" table, which falls apart when you have a category, or requirement, with no parent (other than the project itself). Having just a category table, and having category 1, cagegory 2, category 3, and category 4 rows in my requirements table, which once again falls apart because it has no way of showing parent relationships for the categories *sigh*. I'm sure I should know this but my brain doesn't want to work right now. I hope that explains it somewhat. If not, feel free to e-mail me @ URL and I'll even send you the database hah.

Any help would be greatly appreciated!!
Thank you.

Pat

There are several threads involving this category / sub category issue that show several different methods of doing it. Search and see if you can find one. I would probably do it something like this. (and this is only one way)

table.projects
ID name
1 My Project

table.categories
ID parent category
1 0 label1
2 1 label2
3 0 label3

(label2 is a subcategory of label1)

table.requirements
ID project_ID category_ID requirement
1 1 2 Make it work right

(requirement is My Project and falls in the label2 category)

Wow, thanks for the quick reply. I was getting confused for some reason about the category's parent. This head cold's gettin to me.

Thanks again

Pat