August 2009
Intermediate to advanced
893 pages
26h 48m
English
Content preview from Database Systems: Concepts, Design and Applications
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7.2. Superclass and Subclass Entity Types
As it has been discussed in Chapter 6, Section 6.2.1, an entity type is a set of entities of the same type that share the same properties or characteristics. Subclasses (or subtypes) and superclasses (or supertypes) are the special type of entities.
Subclasses or subtypes are the sub-grouping of occurrences of entities in an entity type that is meaningful to the organisation and that shares common attributes or relationships distinct from other sub-groupings. Subtype is one of the data-modelling abstractions used in EER. In this case, objects in one set are grouped or subdivided into one or more classes in many systems. The objects in each class may then be treated differently in certain circumstances. ...