Information Modeling and Relational Databases, 2nd Edition

9.6. Subtyping

Both UML and ORM support subtyping, using substitutability (“is-a”) semantics, where each instance of a subtype is also an instance of its supertype(s). For example, declaring Woman to be a subtype of Person entails that each woman is a person, and hence Woman inherits all the properties of Person. Given two object types, A and B, we say that A is a subtype of B if, for each database state, the population of A is included in the population of B. For data modeling, the only subtypes of interest are proper subtypes. We say that A is a proper subtype of B if and only if A is a subtype of B, and there is a possible state where the population of B includes an instance not in A. From now on, we’ll use “subtype” as shorthand for “proper ...