B.2: Supertypes, Subtypes and Relationships
If a supertype participates in a relationship, then all of its subtypes also participate in that relationship. We say that a supertype’s relationships are inherited by its subtypes. The converse is not true: if the model specifies specifically that a subtype participates in a relationship, then its siblings (other entity types that are subtypes of the same supertype) do not participate in that relationship.
As an example, consider that members can borrow items (i.e., any item of any type) from the library but only books have authors. Our model can be extended as follows:
This model excludes the database from storing an author of a magazine or that a video has an author, but the model allows videos, magazines, and books to be borrowed by members.