The idea is that several entities with common attributes can be generalized into a higher-level supertype entity or, alternatively, a general entity can be decomposed into lower-level subtype entities.
- The purpose is to attach attributes to the proper level, and thus avoid having a large number of null values.
- An arrow is directed from the subtype to the supertype entities.
manageris a subtype entity to the supertype entityemployee.
Two approaches to reconfigurate the generalization hierarchies into simpler concepts:
- Collapse a one-level generalization hierarchy of subtype and supertype entities into a single entity, by adding all attributes of the subtype entities to the supertype entity.
An additional attribute must be added to this single entity that will discriminate among the various types. - Retain the supertype entity and all subtype entities as full entities and create explicit named relationships to represent the ISA (→) relationships