9.3 Entity Subtypes and Supertypes

Inheritance between entities is supported by the E-R model. When an entity E1 inherits from entity E2, the following applies:

		All attributes and relationships that apply to E2 can also apply to E1 but not vice versa.
		E1 is a subtype of E2.
		E2 is a supertype of E1

For example,

		A Member entity can be a subtype of a User entity.
		Attributes such as Name, DOB which are part of the supertype User entity can apply to the subtype Member.
		However, an attribute Library_ID of the Member entity is not part of the User entity.
		Not every user has a library id number, only member users do.