This tip may seem a bit silly, but I've seen people bitten by this, so I thought I would mention it. I'm sure you're aware of this, but just in case ...

In a properly normalized database, data should usually be represented in one and one one place. As a result, when creating an order_item table, it seems to make sense to merely provide the product id in that table and lookup the price via a join. Unfortunately, this doesn't work. The price at the time of the sale (and perhaps other things) needs to be stored directly in the order_item table lest the customer be charged a different amount due to a price change. This is not denormalizing the database because price at the time of sale is similar, but not identical to, the price of a given item.

Cheers,
Ovid

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