Information Modeling and Relational Databases, 2nd Edition

7.2. Occurrence Frequencies

Let n be some positive integer. To indicate that each entry in a fact column must occur there exactly n times, the number n is written next to the role (see Figure 7.1). This is an example of a frequency constraint (also called an occurrence frequency constraint). If n = 1, this is equivalent to a uniqueness constraint—in this case the usual bar notation for uniqueness should be used instead of a “1” mark.

Figure 7.1. A simple occurrence frequency constraint on role r.

Like uniqueness constraints, frequency constraints are local constraints on the role(s), not global constraints on the object type. The constraint ...