Correct, it is basic data model design in that it is only for handling basic data models. When you are dealing with complicated data models with many contingenc

Correct, it is basic data model design in that it is only for handling basic data models. When you are dealing with complicated data models with many contingencies, you aren't always able to have a data model incapable of representing invalid states. Sometimes it's necessary to rely on application logic/stored procedures/index constraints, etc, in order to enforce valid state.