Sounds like you could use Indirect Set Analysis. There is a qvw available that shows a lot of set analysis samples, including Indirect Set Analysis.
Basically you can select values based on a sub selection.
For instance (from the mentioned Set Analysis.qvw)
sum({$<CompanyName = p({$<CategoryName={'Baby Clothes'}>} CompanyName)>} Sales)
Sales for Customers who purchased Baby Clothes.
The set p({}) returns the values of CompanyName (Customer) that purchased Baby Clothes. </body>
