In the real world, this would not be allowed, as two or more customers might share the same name. This is legal, and provides a way of relating tuples of distinct relations. If a customer has several accounts, we must duplicate her or his address for each account. If a customer has an account but no current address, we cannot build a tuple, as we have no values for the address. We would have to use null values for these fields.
Null values cause difficulties in the database.