 <p>Ok, you have some serious issues here. Orders have mulitple items, invoices have multiple orders and payments may apply to mulitple orders and invoices. Orders may appear on multiple invoices (if they don't pay right aways which is common).</p> <p>So what you need are linking tables. You should start with an ORDERINVOICE table which has both the order id and the invoice ID. Then an ORDERPAYMENT table with paymentid and Order id. </p> <p>You also need to consider that in an ordering situation, you must record the details of the order as it occurred at the time. That means that while you should have the user_id to link to the current user, you should record the user's name, billing address and shipping addres as it was at the time of the order. You will need this information later to deal with any questions on the order. Further you need to ensure that you store the details of the order in a separate table called ORDERDETAILS which store the indivdual line items, the price at the time of the order and the name of the item ordered. You will need this for accounting reasons. You do not under any cuircumstances want to rely on a join to a product table to figure out the price of an order in the past. This will cause your finanacial records to be inaccurate.</p>