Interesting. Your syntax is clean and intuitive. Playing devil's advocate, in thinking about sys.database_principals where groups and users exist together and both share the same set of surrogate keys, I am wondering if the filtered FK feature would be compensating for a flawed data model. A feature like this also seems like it would affirm the violation of the idea that a column should represent only one type of entity. Have you looked at ANSI-SQL standards to see if a similar feature existed?