How To Conditionally Count Distinct Values?

Here is my dataset used by my report definition. The combo of barcode and order id is unique. The 'isDiscountedItem' field indicates if the customer used a coupon to purchased an item at a lower price.

departmentId classId barcode orderId isDiscountedItem
----------------------------------------------------------------------------------------------------------------------
1 1 123 1 True
2 7 456 1 False
1 1 123 2 False
1 1 123 3 True
1 1 789 3 True
2 7 456 3 False
... ... ... ... ...

I want to group my report by department id, class id and barcode. Then, I want to count all distinct order ids for which there was at leat one discounted item.

My report would produce the following output considering the above dataset:






Merchandise Number of customers who used a coupon
--------------------------------------------------------------------------------------------------------------
Department 1 2

Class 1 2

Barcode 123 2
Barcode 789 1
Department 2 0

Class 7 0

Barcode 456 0


I've been looking at a possible solution using hash tables defined in the report code but I would like to find a 'cleaner' solution. Any help would be appreciated.