1583. Lock row before balance update — Helpdesk — implementation variant 33

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

In a Helpdesk application, implement this production problem: Prevent two workers from spending the same account balance concurrently. The API/service receives user input and must return or modify data safely. Give the SQL solution, explain why it works, and show how the same operation is called from the selected stack.

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Answer & Explanation

4 Answers

Verified

Option A

SELECT * FROM the_table;

Additional Explanation

Incorrect: it does not implement the stated business rule and may modify/fetch the wrong rows.

Verified

Option B

UPDATE the_table SET status='active';

Additional Explanation

Incorrect: it does not implement the stated business rule and may modify/fetch the wrong rows.

Verified Correct Answer

Option C

START TRANSACTION; SELECT id,balance FROM accounts WHERE id=:account_id FOR UPDATE; UPDATE accounts SET balance=balance-:amount WHERE id=:account_id AND balance>=:amount; COMMIT;

Additional Explanation

FOR UPDATE locks the selected row until commit/rollback in supported transactional engines. Keep the transaction short.

Verified

Option D

DELETE FROM the_table;

Additional Explanation

Incorrect: it does not implement the stated business rule and may modify/fetch the wrong rows.