The GATE Grind

GATE 2025 DA – Question 56

Database Management and Warehousing · Integrity constraints and normal forms · 2 marks · Multiple select

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?

  1. aadhaar is a candidate key in the Customer relation
  2. phone can be NULL in the Customer relation
  3. aadhaar is a candidate key in the Person relation
  4. aadhaar can be NULL in the Person relation

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Show answer and explanation

Correct answer: (B) phone can be NULL in the Customer relation; (C) aadhaar is a candidate key in the Person relation

Explanation

In Customer the primary key is email, and aadhaar is only a foreign key. Several customers could share the same aadhaar, so it need not be unique and is not a candidate key (A is false). The phone column has no NOT NULL constraint, so it can be NULL (B is true). In Person, aadhaar is the primary key, so it is a candidate key (C is true) and it cannot be NULL (D is false).