Explain the WHEN clause on a trigger

On a FOR EACH ROW trigger, WHEN (condition) decides per row whether the body executes. It references NEW/OLD without the colon, can use simple SQL functions but not PL/SQL functions or subqueries, and is cheaper than an IF at the top of the body because it avoids entering PL/SQL when the condition is false. Use it for the common 'only when this column actually changed' or 'only for this type' guard.

CREATE OR REPLACE TRIGGER trg_salary_raise_audit
  AFTER UPDATE OF salary ON employees
  FOR EACH ROW
  WHEN (NEW.salary <> OLD.salary)          -- no colon here
BEGIN
  INSERT INTO salary_audit(employee_id, old_salary, new_salary, changed_on)
  VALUES (:OLD.employee_id, :OLD.salary, :NEW.salary, SYSDATE);
END;
/