Generally, the downside of denormalizing is that you risk inconsistency, and some extra storage. The inherent assumption in this case is that the customer ID (o

Generally, the downside of denormalizing is that you risk inconsistency, and some extra storage. The inherent assumption in this case is that the customer ID (or whatever the key is) doesn't change for any associated table, and in that world, inconsistency is not a big risk. So yea, you store some extra data but if you've designed your ids well, it's not that big of a cost.