978. Prevent duplicate active subscription — trigger case 2

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

Implement the database trigger for this integrity/audit requirement. Consider NULL-safe comparisons and transaction behavior.

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Answer & Explanation

4 Answers

Verified Correct Answer

Option A

DELIMITER // CREATE TRIGGER trg_subscription_validate BEFORE INSERT ON subscriptions FOR EACH ROW BEGIN IF EXISTS(SELECT 1 FROM subscriptions WHERE customer_id=NEW.customer_id AND status='active') THEN SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT='Active subscription already exists'; END IF; END // DELIMITER ;

Additional Explanation

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

Verified

Option B

Use a trigger that recursively updates the same table without safeguards.

Additional Explanation

This option is incorrect for the stated requirement.

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Option C

Use a foreign key for an audit event that has no parent key.

Additional Explanation

This option is incorrect for the stated requirement.

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Option D

Use a SELECT-only query and do not enforce the rule.

Additional Explanation

This option is incorrect for the stated requirement.