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.

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