oracle语句问题

呵呵，你把自己搞糊涂了，先编个号吧
1、
select distinct department_id from departments d
where d.department_id not in (select distinct e.department_id from employees e where
e.department_id is not null);

2、
select distinct department_id from departments d
where (select distinct e.department_id from employees e
where e.department_id is not null);

3、select distinct department_id from departments d
where not exists (select distinct e.department_id from employees e
where e.department_id <> null);

4、
select distinct e.department_id from employees e
where e.department_id is not null

5、
select distinct e.department_id from employees e
where e.department_id <> null

首先 2 是可以的
因为你的数据库中 4 是有数据的，再用not exists，那么 2 当然没数据了。
再说 3 为什么可以
因为 5 中e.department_id <> null是个不成立的条件，5 永远不存在数据，没有列可以等于null，若为空值必须用is null或者用nvl函数，否则只能为e.department_id ＝ ‘null’，所以再用not exists，就