In reference to the lookup table their will be both enteries in the subscriber table and the customer table. Their will also be multiple enteries for both - that is the invoice table could contain id for both the subscriber table and the customer table. The invoice table contains an invoice id as well as an id for the either the subscriber or the customer. The invoice id is unique for each invoice however. The lookup table needs to select the invoice number, either the subscriber id or customer id, as well as the address from either the subscriber or customer table based on the id in the invoice table - that is the customer or subscriber id. Yes you are correct that the id will never be the same for the subscriber or customer. If I use a select statement based on a large record set will this likely be slow. This is an application which needs to be fast because the users are on the phone helping clients. In that case would I be better off storing the data I need in a separate lookup table.