Which statement about primary keys is true?

Which statement about primary keys is true?

• A. They can have duplicates.
• B. They can be null.
• C. They enforce entity integrity and uniqueness.
• D. They are used to improve lookup speed.

Answer: (C)

Primary keys are the unique identifier for each row in a table, ensuring that every record has a distinct identity and that no two rows share the same value. This directly enforces entity integrity and uniqueness, since each row can be precisely identified and referenced by others (through foreign keys) without ambiguity. While primary keys are often backed by an index to speed up lookups, the essential point is the guaranteed uniqueness and presence of a value for every row. The other statements describe properties that don’t apply to primary keys: duplicates are not allowed, and nulls are not permitted.

Primary keys are the unique identifier for each row in a table, ensuring that every record has a distinct identity and that no two rows share the same value. This directly enforces entity integrity and uniqueness, since each row can be precisely identified and referenced by others (through foreign keys) without ambiguity. While primary keys are often backed by an index to speed up lookups, the essential point is the guaranteed uniqueness and presence of a value for every row. The other statements describe properties that don’t apply to primary keys: duplicates are not allowed, and nulls are not permitted.