Information Modeling and Relational Databases, 2nd Edition

5.6. Summary

For a given state of the database, the population of a role r, pop(r), is the set of objects referenced as playing that role in that state. A ∪ B, the union of sets A and B, is the set of all elements in A, B, or both. The null set or empty set, { }, has no members. The population of an entity type A, pop(A), is the union of the populations of its roles. Roles of an elementary fact type are called fact roles. A relationship type used purely to identify or reference some object is a reference type; its roles are called reference roles. For a given state, the population of a fact type (stored or derived) is its set of fact instances.

Step 5 of the conceptual schema design procedure requires us to add mandatory role constraints and ...