Information Modeling and Relational Databases, 2nd Edition

7.3. Ring Constraints

When two roles in a predicate are played by the same object type, the path from the object type through the role pair and back to the object type forms a “ring”. If the roles are played by subtypes with a common supertype, the path from and back to the super-type also forms a ring. The role pair typically forms a binary predicate, but may be part of a longer predicate (see the shaded role pairs in Figure 7.7) or may even come from different predicates in a join path (join constraints are discussed in Chapter 10). A ring constraint may apply only to a pair of roles like this.

Figure 7.7. Ring constraints may apply to the shaded role pairs.

Before discussing the various kinds of ring constraint, some standard definitions ...