MySql---where---in条件过多导致的效率缓慢问题的优化方法

更新一张表中的某些记录值，更新条件来自另一张表(数据量大)，效率极其低下。

update A set A.a = 0 where A.b in
(
 select distinct B.b from B where B.c = 0
);

优化后

 update A set A.a = 0 where A.b in (
  select b from (select distinct B.b from B where B.c = 0) as co
 );

总结：对于where xxx in 子句效率极其低下问题，经过in的子句外包装一层select xxx from ( ... ) as co 后，极大优化效率。