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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.
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