>These are po numbers that customers give us...so there are no guarantees about uniqueness. As a matter of fact, we have gotten a duplicate PO number within a customer (from a major retailer...I wouldn't expect that to happen). The only time we retrieve an invoice (or open order) this way is when a store calls and says "Where is po number 12345?"

Sorry if this has already been covered, but can you just keep the CustID and PoNum indexes, and do:

SELECT * FROM PastInvoices WHERE CustID = ?lcCustID AND PoNum = ?lcPoNum