Information Modeling and Relational Databases, 2nd Edition

4.4. External Uniqueness Constraints

The uniqueness constraints discussed so far are called internal (intrapredicate) uniqueness constraints, since each applies to one or more roles inside a single predicate. We now discuss external (interpredicate) uniqueness constraints. These apply to roles from different predicates. To help understand these constraints, and prepare for later discussion on queries, the conceptual join operation is first discussed.

Consider the schema and sample population shown in Figure 4.31. The left-hand binary fact table indicates that employee 15 drives two cars, with registration numbers PKJ123 and ABC000. The right-hand binary fact table tells us that the car PKJ123 is imported from Italy. The ternary fact table indicates ...