How to provide DISTINCT functionality in WCF

I have WCF Data service on top of EF (old-fashioned edmx; not code-first) that has millions of rows of data. I am restricting result set to 500 rows. So far, so good!

I want to give the consumer the ability to "drill" into the data. Ideally, consumer code would run something like select distinct Province from my service, user would choose the state, and then code would run select distinct Zip where Province == p (my business domain is very different, but hopefully you get the picture!).

Obviously, I can't get everything and run distinct on the client side. How do I provide this ability in the service? I don't have EntitySet "Provinces" or "Zips" exposed in the service. Should I extend MyEntities class, and try to simulate those sets there? Or is there a simpler way to expose other collections, besides the ones that are automatically exposed from Entity Framework?

I hope the question makes sense...

UPDATE: I think I caused the confusion when I mentioned that I was using WCF service. I actually am using WCF Data Service trying to provide OData wrapper over Entity Framework 4开发者_如何学编程.1. Hopefully, this clarifies the question...

Have you considered dropping your supposedly propietary interafce and going standards?

It is quit easy to expose ODATA from .NET and that allows the user to drill down. THen all you need are some transformations to expose elements like Province or Zip as in a sane data model and there you go ;)

The good thing about OData is that it allows you to easily pass LINQ to the backend and do projections (get me all proovinces from customers where customer name starts with "a") and doesn ot necessarily deal with compelte entities.