Database Modeling and Design

2.2 Advanced ER Constructs 23
2.2 Advanced ER Constructs
2.2.1 Generalization: Supertypes and Subtypes
The original ER model has been effectively used for communicating fundamental
data and relationship definitions with the end user for a long
time. However, using it to develop and integrate conceptual models with
different end user views was severely limited until it could be extended
to include database abstraction concepts such as generalization. The generalization
relationship specifies that several types of entities with certain
common attributes can be generalized into a higher-level entity
type—a generic or superclass entity, more commonly known as a supertype
entity. The lower levels of entities—subtypes in a generalization hierarchy—can
be either disjoint or overlapping subsets of the supertype
entity. As an example, in Figure 2.4 the entity Employee is a higher-level
Employee
supertype
subtypes
d
Manager
Engineer
Technician
Secretary
(a) Generalization with disjoint subtypes
Individual
O
Employee
Customer
(b) Generalization with overlapping subtypes and completeness constraint
Figure 2.4
Supertypes and subtypes