GATE DA 2025 Set 1 — Question 56

MSQ+2 / -0EasyKey ConstraintsRelational ModelDatabase Management & WarehousingCandidate & Super KeysNormalization

Database Management & Warehousing → Normalization → Candidate & Super Keys

Last updated 5 September 2026

Question

Consider the following two relations, named Customer and Person, in a database: Person ( aadhaar CHAR(12) PRIMARY KEY, name VARCHAR(32)); Customer ( name VARCHAR(32), email VARCHAR(32) PRIMARY KEY, phone CHAR(10), aadhaar CHAR(12), FOREIGN KEY (aadhaar) REFERENCES Person(aadhaar)); Which of the following statements is/are correct?

Correct answer (B) phone can be NULL in the Customer relation; (C) aadhaar is a candidate key in the Person relation Solution Option A: In Customer, aadhaar is a foreign key but there is no UNIQUE constraint on it. A person could theoretically have multiple customer records (e.g., different emails). Thus, it is not necessarily a candidate key. Option B: The phone attribute in Customer does not have a NOT NULL constraint, so it can accept NULL values. This is correct. Option C: In Person, aadhaar is the PRIMARY KEY. By definition, a primary key is a candidate key. This is correct. Option D: A PRIMARY KEY attribute cannot be NULL. Thus, aadhaar cannot be NULL in Person . Turn this into a strength. Explore AI-powered practice and doubt support with Success Tracker.

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Correct answer (B) phone can be NULL in the Customer relation; (C) aadhaar is a candidate key in the Person relation Solution Option A: In Customer, aadhaar is a foreign key but there is no UNIQUE constraint on it. A person could theoretically have multiple customer records (e.g., different emails). Thus, it is not necessarily a candidate key. Option B: The phone attribute in Customer does not have a NOT NULL constraint, so it can accept NULL values. This is correct. Option C: In Person, aadhaar is the PRIMARY KEY. By definition, a primary key is a candidate key. This is correct. Option D: A PRIMARY KEY attribute cannot be NULL. Thus, aadhaar cannot be NULL in Person .

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