I recently had fun with an all-to-common issue with SQL driven websites: hierarchical data. For those who don't like big words, think trees. Other people have already discussed storage methods, and I would actually highly suggest you read the writeup if you haven't already.

He includes a sample table you can work from and some code that can be used to generate a select (based on a $depth variable) parent/child values from it. A few simple function calls later and you have a nested array or lots of little subarrays maintaining the parent/child relationships the database has.