3300. Race condition: Prevent two users from claiming the same limited coupon. — Testing & QA

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

In a race condition module, solve this as a production problem using Testing & QA: Prevent two users from claiming the same limited coupon. Provide the SQL, explain the logic, show safe application integration, and mention failure/edge cases.

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Answer & Explanation

4 Answers

Verified

Option A

SELECT * FROM table_name;

Additional Explanation

Incorrect: it does not satisfy the stated business rule and can return/change unintended data.

Verified Correct Answer

Option B

START TRANSACTION; UPDATE coupons SET claimed_by=:user_id,claimed_at=NOW() WHERE id=:coupon_id AND claimed_by IS NULL; COMMIT;

Additional Explanation

The conditional update is atomic at the row level; check affected rows to know whether the claim succeeded.

Verified

Option C

UPDATE table_name SET status='active';

Additional Explanation

Incorrect: it does not satisfy the stated business rule and can return/change unintended data.

Verified

Option D

DELETE FROM table_name;

Additional Explanation

Incorrect: it does not satisfy the stated business rule and can return/change unintended data.