not in 与 left join 的结果不同，可能是什么原因哦，解决马上给分
select * from a where id not in (select distinct id from b)
返回空集
select * from a left join b on a.id=b.id where b.id is null
返回很多
有点迷惑，还没找到原因，大虾给指点下啊
[解决办法]
select * from a where id not in (select distinct id from b)
改成：
select * from a where id not in (select distinct id from b WHERE id IS NOT NULL)
[解决办法]
a表或b表中的id有為null值？
