Question

A supertype/subtype hierarchy is which of the following?

(a) Each subtype has only one attribute

(b) Each supertype has only one attribute

(c) Each subtype has only one supertype

(d) Each supertype has only one subtype

This question was posed to me in a national level competition.

This intriguing question comes from The Atomic Data Models and Business Models topic in section Planning Oracle Applications of Oracle

Answer 1

Right answer is (c) Each subtype has only one supertype

Easy explanation: Two entities are of distinct types if they have no attributes in common. It is possible for entities to have both common and distinct attributes. If they have a common identifier (that is, a common primary key), they have a special supertype-subtype relationship: they are neither distinct nor the same.