Consider a database with three tables, Parent, ChildTypeA, and ChildTypeB. Parent has the fields id, and child_type (an ENUM of “a” and “b”), and a unique constraint over both fields. ChildTypeA and ChildTypeB both have the field supertype_key (the aforementioned ENUM), and parent_id. They also have a composite unique foreign key of (parent_id, supertype_key) to (Parent.id, Parent.child_type).