Calculating unique sums

I have a transaction table: transaction_id,product_id,customer_id. I want to get list of UNIQUE customers (not # of transactions) that purchased each product.
The result may look something like this: Product Customer Count

A           500
B            23
C           175

Does anyone have advice on the most efficient way to do this? I am dealing with hundreds of millions of transactions.

Thanks Received on Tue Nov 28 2000 - 14:31:14 CST