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GATE DA 2025 Q56: Keys and NULLs in Person and Customer tables

Question

Consider the following two relations, named Customer\texttt{Customer} and Person\texttt{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?

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Select all correct options. No negative marking, no partial marks.
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Answer: B, C (official GATE 2025 final answer key)

Concept used: Primary keys, foreign keys, uniqueness and NULL constraints in SQL.

First step: A primary key is unique and NOT NULL. In Customer, aadhaar is only a foreign key with no UNIQUE constraint, and phone has no NOT NULL constraint.

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Source: official GATE 2025 DA question paper and final answer key, organised by IIT Roorkee. Question paper · Answer key

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