Is it possible to represent a ‘mutual IS-A’ relationship in an entity-relationship diagram? Is the following scenario correct and why:


A and B are two entities
B is-a A 
A is-a B  


And what if A, B were relationship?
In databases, subtyping means that one domain is a subset of another. If A is a subset of B and B is a subset of A, then it means A = B. The same logic applies whether A and B are entity sets or relationship sets. There would be no point distinguishing A and B in an ER model.