GATE CS 2021 Set 2 Q.16 (CS Q.6): ER Model and Relational Schema Design

Consider the following statements $S_1$ and $S_2$ about the relational data model:

$S_1$ : A relation scheme can have at most one foreign key.

$S_2$ : A foreign key in a relation scheme $R$ cannot be used to refer to tuples of $R$ .

Which one of the following choices is correct?

Both $S_1$ and $S_2$ are true.

$S_1$ is true and $S_2$ is false.

$S_1$ is false and $S_2$ is true.

Both $S_1$ and $S_2$ are false.

Official answer and solution

Official answer: D
From: Official GATE 2021 answer key

Solution

Evaluate standard relational database definitions regarding foreign key constraints and self-referential relationships.

Statement $S_1$ states that a relation scheme can have at most one foreign key. This is false. A relation scheme can contain multiple foreign keys referencing candidate keys in different relations or the same relation (for instance, an Enrollment table referencing both Student and Course).

Statement $S_2$ states that a foreign key in a relation scheme $R$ cannot be used to refer to tuples of $R$ . This is also false. A foreign key can reference the primary key of its own relation, known as a self-referencing foreign key or recursive foreign key (for instance, an Employee table where attribute manager_id references emp_id of the same Employee table).

Therefore, both statements $S_1$ and $S_2$ are false, which corresponds to option D.

Final answer

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2021 Computer Science and Information Technology - Session 2 (CS2)
Question: 16 of 65, printed as CS Q.6
Section: Computer Science and Information Technology

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