Referential integrity, or foreign key constraint, is one of the basic relational data model constraints. It’s objective is to maintain data of two base relations in consistent state during tuple insertion, deletion, and modification. In the course of third normal form database schema design, there arise situations where it is not possible to define a referential integrity between two relation schemes, although the real system business rules dictate the definition of an appropriate database constraint. The problem arises because of the fact that the referencing relation scheme contains only the proper subset of the referenced relation scheme primary key. To overcome the described problem, the notion of extended referential integrity was introduced in the paper. The extended referential integrity is a constraint that spans more than two relation schemes. Accordingly, checking the satisfaction of that constraint involves the relations over all the included relation schemes. All the properties of the extended referential integrity, as described in the paper, correspond to the ANSI SQL/2 standard.