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Exhibit 18-16. Associative entity and cardinality violation.
Between the two previous spectrums are several suspicious cardinality
patterns. The use of mandatory optionality on many-to-many relation-
ships, though theoretically possible, still implies a degree of mutual depen-
dencies that may not map closely to business realities. In addition, another
unlikely situation is the one-to-many recursive relationship with manda-
tory minimum cardinality.
It is useful in data model reviews to quickly look for telltale signs that
indicate error conditions. Experience has shown that such signs help jump-
start the review process to uncover the business issues lying beneath the
surface rather than simply correcting cardinality errors.
An associative entity is an entity
that is created when information is needed about a relationship or when
representation is needed to resolve a many-to-many relationship between
two entities. In this article, the entities through which the associative
entity was created are called the parent entities of the associative entity.
Associative Entity and Relationships.
Certain rules exist regarding the cardinality of entities involved in
an associative entity situation. First, an associative entity exists because of
other entities, and it does not exist without the relationship of at least two
other entities (see Exhibit 16 for the correct use of cardinality). Second, an
associative entity is used only to resolve many-to-many relationships; in
fact, associative entities can be involved in any combination of cardinality
as long as there is a need to keep information about the relationship. In the
example shown in Exhibit 17, the attribute Sales Price of the painting could
be an attribute neither of the Artist nor of the Live Model entity.
Rules.
Third, if the identifiers of the two related fundamental entities do not
provide uniqueness for the associative entity, the entity is still considered
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