Subtype Definitions and Subtype Sets

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If a supertype has more than one possible subtype, you can often define rules concerning the mandatory or possible use of subtypes.

For instance, suppose a Person table has five subtypes, i.e. Male, Female, Student, Professor, Office Staff.

Subtypes belong together in groups called subtype sets. In the example, Male and Female belong to the Gender group, and Student, Professor and Office Staff belong to the Occupation group. In this example subtypes are mutually exclusive with regard to subtype sets. In general, it is possible for subtype sets to overlap, i.e. a subtype may belong to more than one subtype set.

The purpose of defining subtype sets is to formulate exclusivity rules and totality rules that apply to the members of the subtype set.