Avoiding Pitfalls in Data Modeling - High-Performance Web Databases: Design, Development, and Deployment - page 282

Exhibit 18-7. Multiple supertypes.

conflicting attributes being inherited by the subtype will prompt the mod-
eler to address the problem. This is another reason why key attributes
should be identified as soon as possible.

Sometimes it is understood that two entities are
subtypes of something of a higher order , but the business has no need to
keep information about the entity at the generalized level (e.g., customer
and vendors could both be people, but the company may have no desire to
keep information about people in a generic way).

Omission of Supertype.

In other situations, however, the omission of the supertypes could cause
problems. The examples in Exhibit 8b show a supertype engaged in an
associative Entity Relationship. The omission of the supertype, as shown
in Exhibit 8a, violates the cardinality rule of entities as parent of an asso-
ciative entity. This rule states that an associative entity by definition has
relationships with at least two other entities, and these relationships can-
not be optional.

The example shown with the supertype omitted (Exhibit 8a) does not
accurately represent the business rules because the cardinality of both of
the relationships linking Project Assignment with Regular Employee and
Seasonal Employee entities is optional. In this case, there is no guarantee