I'm stumped by a validation problem I'm having.
I have a simple table that has the fields:
All invoices are entered into this table and I need a way of validating that an invoice isn't entered twice for the same supplier by mistake.
I want the invoice number to be unique to the supplier only
(i.e: one invoice number could be repeated many times throughout the table, but is only allowed once for each Supplier ID).
I hope I've explained this clearly enough - any help on how this could be achieved would be greatly appreciated!