43. Unique constraint

DBMS Published 30 Sept 2026 Updated 30 Sept 2026 0 Views Verified

Ensure customer email addresses are unique.

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Answer & Explanation

4 Answers

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Option A

ALTER TABLE customers ADD INDEX email UNIQUE FALSE;

Additional Explanation

This option is incorrect for the stated requirement.

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Option B

ALTER TABLE customers ADD CHECK email UNIQUE;

Additional Explanation

This option is incorrect for the stated requirement.

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Option C

ALTER TABLE customers SET email PRIMARY;

Additional Explanation

This option is incorrect for the stated requirement.

Verified Correct Answer

Option D

ALTER TABLE customers ADD CONSTRAINT uq_customers_email UNIQUE (email);

Additional Explanation

Correct implementation. Prefer parameterized queries in application code, validate affected rows for UPDATE/DELETE, and use transactions for related writes.