Question

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.

• 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.

Hint:

A primary key is unique and not NULL, so (A) is true. A candidate key can be chosen as the primary key, so (R) is false.

Answer 1

The Correct Option is C

Step 1: Read what the Assertion says:
A PRIMARY KEY column identifies each row of a table.
Because of this job, no two rows can hold the same value in it.
A primary key also cannot hold NULL, since NULL means an unknown value and cannot identify a row.
So the Assertion is TRUE.

Step 2: Read what the Reason says:
A candidate key is any column (or set of columns) that could uniquely identify a row.
A table can have several candidate keys.
One candidate key is chosen as the primary key, and the others become alternate keys.
So a candidate key IS eligible to become the primary key. The Reason says the opposite, so the Reason is FALSE.

Step 3: Match with the four options:
Options 1 and 2 need the Reason to be true, but it is false, so both are wrong.
Option 4 says the Assertion is false, but the Assertion is true, so it is wrong.
Option 3 says A is true and R is false, which matches our result.

Final Answer:
Assertion (A) is true, but Reason (R) is false. This is option 3.