The concept of specializations and generalizations allows the formation of entity type subtypes. There are a number of ways in which an entity type can be specialized. The criterion used for the specialization is defined by the specialization category. Specializations are linked by the specialization category to the source entity type (= generalization).
Generalization, specialization category, specialization
People at the university
Students, professors, administrative staff, research assistants
The generalization contains the attributes that are common to all entities of an entity type. These attributes are inherited by the specializations (see alsoAttributes of entity types) and may be supplemented by other specific attributes.
In the university example the generalization People at the university has the attributes Number, Name, and Address of a university member, which it passes on to its specializations (Students, Professors, and so on). The specialization Students also has the following attributes: Matriculation number, Assigned professor, and Course start.
The specialization category can be characterized by the attributes:
Each entity of the generalization occurs in at least one specialization of the category.
Each entity of the generalization occurs in a maximum of one specialization of the category.
The specialization category Activity is complete in the university example, since each university member belongs to at least one specialization. This specialization category is not disjoint, however, since it is conceivable that a person might be at the same time both a student and a research assistant with the result that they would occur in both specializations.
A specialization category need be neither complete nor disjoint.