Which statement appropriately contrasts primary keys and secondary keys?

Which statement appropriately contrasts primary keys and secondary keys?

• A. Primary keys allow duplicates; Secondary keys guarantee uniqueness.
• B. Primary keys enforce uniqueness; Secondary keys improve lookup speed but may duplicate values.
• C. Both always enforce uniqueness.
• D. Secondary keys cannot be used for lookups.

Answer: (B)

Primary keys are the unique identifier for each row and enforce that no two rows share the same value (and they’re typically not null). Secondary keys (indexes on other columns) exist to speed up searches; they don’t require uniqueness and can have duplicate values across rows. So the statement that primary keys enforce uniqueness while secondary keys improve lookup speed but may duplicate values accurately contrasts their roles. The other ideas fail because primary keys don’t allow duplicates, not all keys require uniqueness, and secondary keys are indeed used to speed lookups.

Primary keys are the unique identifier for each row and enforce that no two rows share the same value (and they’re typically not null). Secondary keys (indexes on other columns) exist to speed up searches; they don’t require uniqueness and can have duplicate values across rows. So the statement that primary keys enforce uniqueness while secondary keys improve lookup speed but may duplicate values accurately contrasts their roles. The other ideas fail because primary keys don’t allow duplicates, not all keys require uniqueness, and secondary keys are indeed used to speed lookups.