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When an entity instance must be a member of only one sub-type, it is which of the following?

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When an entity instance must be a member of only one sub-type, it is which of the following?

(a) Disjoint with total specialization

(b) Disjoint with partial specialization

(c) Overlap with total specialization

(d) Overlap with partial specialization

This question was addressed to me during an online interview.

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

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The correct option is (a) Disjoint with total specialization

The explanation is: GradStudent (_id_, name, gradStuff) UndergradStudent (_id_, name, underGradStuff) Professor (_id_, name) Teaches(_prof_id_, _stud_id_)

Considering that the relational database above is meant to represent total disjoint specialization, i.e. there is no table Student but rather two completely separate ones, when writing this in SQL in order to implement the database, how would I fetch the student id for the Teaches table? I cannot figure out how to make a foreign key from two different tables.

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