A Closer Look at the Join-Equality Constraint

摘要

Assume an ORM-diagram where there exist two (or more) paths from one entity type to another through a sequence of many-to-one binary facts. Here we may have a join-equality constraint saying that if we follow the different paths from an instance of the first entity type, we should find the same instance of the second entity type. This constraint is inherent in most ticketing/reservation systems, where it may give rise to overlapping foreign keys in the relational database. Another interesting observation is that if a relation is in 3NF, but not in BCNF, there must be a join-equality constraint in the underlying model.