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SQLBeginner#aggregates#nulls
SQLIntermediate#triggers
What is a trigger?
A block of SQL that runs automatically BEFORE or AFTER an INSERT, UPDATE or DELETE on a table. Useful for auditing and derived columns, but heavy triggers hide logic and hurt performance.
Example
CREATE TRIGGER audit_salary AFTER UPDATE ON employees FOR EACH ROW INSERT INTO salary_audit(emp_id, old_sal, new_sal, changed_at) VALUES (OLD.id, OLD.salary, NEW.salary, NOW());