Specializations/Generalizations
The aim of the concept of specializations and generalizations is to allow the subdivision of entity types into subtypes. There are a number of different categories of specialization. The specialization category determines the criterion for the specialization. Specializations are linked to the source entity type (=generalization) via the specialization category.
Generalization, specialization category, specialization
|
Term |
Example |
|---|---|
|
Generalization |
People at the university |
|
Specialization category |
Activity |
|
Specializations |
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 (more information: Attributes 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:
-
Complete:
Each entity of the generalization occurs at least once in a specialization of the category.
-
Disjunct:
Each entity of the generalization occurs in a maximum of one specialization of the category.
ExampleThe specialization category Activity is complete in the university example, since each university member belongs to at least one specialization. This specialization category is not disjunct, 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.
NoteA specialization category does not need to be either complete or disjunct.