UPDATE a
   SET (name, age) = (SELECT b_name, b_age
                        FROM b
                       WHERE b.b_idnum    = a.idnum
                         AND b.b_location = '서울'
                      )
 WHERE a.idnum = 450
   AND EXISTS (SELECT 0
                 FROM b
                WHERE b.b_idnum    = a.idnum
                  AND b.b_location = '서울'
               )
;
MERGE INTO a
USING
(
SELECT *
  FROM b
 WHERE b_idnum    = 450
   AND b_location = '서울'
) b
ON (a.idnum = b.b_idnum)
WHEN MATCHED THEN
UPDATE
   SET a.name = b.b_name
     , a.age  = b.b_age
;
-- http://gurubee.net/article/79308