Just a warning, Christina, if your data is set up the way I think it is, t.brown's solution will not immediately give you the answer you're looking for. You will definitely have to go tweaking in order to find the best way to do this.

However, if you're able to give us table DDL and an insert statement with sample data, and let us know what code you've tried to make this work, we'll help you tweak until you get it right.