oracle批量更新

UPDATE t set t.m = (SELECT a.m FROM a WHERE a.n = t.n);
--检查错误方法
SELECT a.n, COUNT(1)
  FROM a, t
 WHERE a.n = t.n
 GROUP BY a.n
HAVING COUNT(1) > 1;
--修改方法考虑业务逻辑
UPDATE t
   set t.m = (SELECT a.m
                FROM a
               WHERE a.n = t.n
                 AND ROWNUM = 1);