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A totality rule for
a subtype set means that ALL supertypes must be subtypes of AT
LEAST ONE of the members in the subtype set. An example could be
that all persons in the information system must have an occupation
(student, professor, or office staff). Another example could be
that all persons have a gender (male, female).
An exclusivity rule for a
subtype set means that each supertype (each person) can only belong
to ONE of the members in the subtype set. For instance, a person
cannot be male and female at the same time.
Rule combinations are
possible. The Gender subtype set is both total and exclusive: each
person has one and exactly one gender.
To the Occupation subtype set,
only the totality rule could apply. This means that all registered
persons are either students, or professors, or members of staff, or
more than one of these. The latter could occur if someone who is a
professor in one department is registered as a student in another
department.
For each person in the
database, both gender AND occupation data exist since both subtype
sets are subject to the totality rule.
Related Topics
Example of Supertypes and
Subtypes
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