Q21
1 markSection A

Assertion (A) : The PRIMARY KEY constraint in SQL ensures that each value in the column(s) is unique and cannot be NULL.

Reason (R) : Candidate keys are not eligible to become a primary key.

Database Query using SQL
Primary Key and Candidate Keys

Options

(A)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A).
(B)Both Assertion (A) and Reason (R) are true and Reason (R) is not the correct explanation for Assertion (A).
(C)Assertion (A) is true, but Reason (R) is false.
(D)Assertion (A) is false, but Reason (R) is true.
Official Answer

Correct option: (C) — Assertion (A) is true, but Reason (R) is false.

primary keycandidate keyalternate keyunique constraintNOT NULLSQL keysrelational database

Marking Scheme

  • 11 mark for selecting option (C); no partial marks in Assertion-Reason questions.

Hint

Recall the key hierarchy: every primary key IS a candidate key that was selected — so candidate keys cannot be 'ineligible' for the role.

Quick Oral Answer

A primary key enforces uniqueness and NOT NULL, and it is always chosen from among the table's candidate keys — so candidate keys are indeed eligible to become the primary key, making Reason (R) false here.

Analysis & Explanation

This question checks whether students correctly understand the relationship between candidate keys and the primary key.


Checking Assertion (A)

  • The PRIMARY KEY constraint in SQL enforces two rules on its column(s): all values must be unique, and no value can be NULL.
  • This is the standard, correct definition of a primary key, so Assertion (A) is true.

Checking Reason (R)

  • A candidate key is any column (or set of columns) that COULD qualify as the primary key because it satisfies uniqueness and non-null requirements.
  • In fact, the primary key is always CHOSEN FROM among the candidate keys — the remaining, unchosen candidate keys become alternate keys.
  • So candidate keys ARE eligible to become the primary key; Reason (R), which claims the opposite, is false.

Conclusion

  • Since (A) is true and (R) is false, the correct option is (C).

Common Mistakes

  1. 1Believing candidate keys and the primary key are unrelated concepts, rather than understanding the primary key is selected FROM the candidate keys.
  2. 2Selecting option (A) or (B) by assuming Reason (R) must be true simply because it sounds like a technical database fact.

Interesting Facts

A table can have multiple candidate keys, but only ONE of them is designated as the primary key; the rest are called 'alternate keys' — exactly the scenario tested earlier in Q12 of this same paper.

Unlike a primary key, a UNIQUE constraint in SQL allows one NULL value per column (in most RDBMS), which is a key practical difference tested in many CBSE database questions.

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Frequently Asked Questions

Can a candidate key become a primary key?

Yes — the primary key of a table is always one of its candidate keys, specifically the one chosen by the database designer to uniquely identify each row.

What happens to candidate keys that are not chosen as the primary key?

Candidate keys not selected as the primary key are called alternate keys; they still satisfy uniqueness but do not enforce the primary key's special role.