update from selected data?

Thanks for all the answers to my last question.

I know I'll turn very red when I get the answer to this one, but I've apparently been staring too long at my books. So, how do I do this in Oracle?

	UPDATE em		; table em


  -->>	  FROM oldEm		; from table oldEm

	   SET em.dept = oldEm.dept
	 WHERE em.dept IS NULL


  -->>	   AND oldEm.id = em.id;

I hope there is something more efficient than

	UPDATE em
	   SET dept = (SELECT dept FROM oldEm WHERE oldEm.id = em.id)
	 WHERE em.dept IS NULL
	   AND EXISTS (SELECT *    FROM oldEm WHERE oldEm.id = em.id);

-- 
Regards,

Hallvard