6.4. Subset, Equality, and Exclusion Constraints
Set-comparison constraints restrict the way the population of one role, or role sequence, relates to the population of another. In an earlier section we considered six ways in which two sets might be related: subset, equality, exclusion, proper subset, overlap, and proper overlap. The last three of these require some objects to exist in at least one of the sets being compared. However, static constraints must apply to every state of the database, including the empty state. So the only set-comparison constraints of interest are subset, equality, and exclusion. These three kinds of constraint are examined in this section.
Suppose a fitness club maintains an information system about its members and ...
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