select dept_id from s_emp
where (select avg(salary)from s_emp group by dept_id)
(select avg(salary) from s_emp)
select dept_id from s_emp
where (select avg(salary)from s_emp group by dept_id)
(select avg(salary) from s_emp)
select avg(salary)from s_emp group by dept_id
这里有group by ,会出现每个部门的平均值,如果有2条记录dept_id不一样,就会出现多行的错误