UPDATE employees SET salary =      
   (SELECT salary FROM employees
   AS OF TIMESTAMP (SYSTIMESTAMP - INTERVAL '2' MINUTE)
   WHERE last_name = 'Chung')
   WHERE last_name = 'Chung';
---------------------------
UPDATE employees
SET salary = (
	SELECT salary
	FROM employees
	AS OF TIMESTAMP (SYSTIMESTAMP - INTERVAL '2' MINUTE)
	WHERE last_name = 'Chung'
)
WHERE last_name = 'Chung';