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Database Systems: Concepts, Design and Applications by S. K. Singh

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7.4. Categorisation

Categorisation is a process of modelling of a single subtype (or subclass) with a relationship that involves more than one distinct supertype (or superclass). Till now all the relationships that have been discussed, are a single distinct supertype. However, there could be need for modelling a single supertype/subtype relationship with more than one supertype, where the supertypes represent different entity set.

For example, let us assume that a vehicle is purchased in a company for transportation of goods from one department to another. Now, the owner of the vehicle can be a department, an employee or the company itself. This is a case of modelling a single supertype/subtype relationship with more than one supertype, where ...

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