There are two additional characteristics between the subtypes and the supertype that need to be addressed, exclusivity and exhaustivity. This is the concept that the an instance of the supertype may or may not be an instance of more than one of the subtypes. This is known as a subtype discriminator. Business Rules Being a database class, we've been focusing primarily on data structures. However even if we model and implement the situation at hand perfectly we still need to address the actual values to be stored.
Business rules are the specifications that preserve the integrity of the logical data model.
Referential Integrity Constraints - Rules governing the relationships between entity types. Domains - What's a valid value for an attribute. Triggering Operations - Other business rules that protect the validity of the attribute values. Our textbook classifies business rules a little more rigorously, splitting them into structural constraints and operational constraints. The structural constraints have to do with the data structure itself.