This week, we learned enhanced entity relationship diagram (EERD). There are two

Question

This week, we learned enhanced entity relationship diagram (EERD). There are two types of constraints between the super and sub types. They are disjointness and completeness constraints. Answer these questions, given this EERD: 1. Is it required that every entity instance in the STUDENT table be associated with an entity instance in the UNDERGRADUATE table or GRADUATE table? Why or why not? 2. Is it possible for an undergraduate student to appear in the UNDERGRADUATE table without appearing in the STUDENT table? Why or why not?

Explanation / Answer

1. No, the completeness for the data model shows a total completeness constraint from STUDENT to the subtypes. However, the total completeness constraint indicates that every instance in the supertypes(STUDENT) must be associated with one row in some subtype ,not all subtypes.

2.

No, Subtypes can only exist within the context of a supertype.

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