SQL：查询作为子查询或CTE写入的最大和数失败

假设表salaries包含employee_id、department_id和salary，如下所示.

e_id | dpt_id | salary
---- | ------ | ------
   1 |      2 |   1000
   2 |      2 |   2000
   3 |      5 |   3000
   4 |      6 |    500
   5 |      5 |    100
   6 |      1 |   1050

我想带来具有最高/最低工资的DPT，我使用了以下查询，但查询失败:

select sum_sal_per_dpt.dpt_id, max(sum_sal_per_dpt.total_salaries)
from (
      select dpt_id, sum(salary) as total_salaries
      from salaries
      group by dpt_id ) as sum_sal_per_dpt
group by sum_sal_per_dpt.dpt_id
union
select sum_sal_per_dpt.dpt_id, min(sum_sal_per_dpt.total_salaries)
from (
      select dpt_id, sum(salary) as total_salaries
      from salaries
      group by dpt_id ) as sum_sal_per_dpt
group by sum_sal_per_dpt.dpt_id

我得到的输出是一个带有dpt_id和工资总和的表，好像只执行子查询:

dpt_id | salary
------ | ------
     2 |   3000
     5 |   3100
     6 |    500
     1 |   1050

而不是

dpt_id | salary
------ | ------
     5 |   3100
     6 |    500

如果我以CTE的方式写，也是一样的.

with
sum_sal_per_dpt as (select dpt_id, sum(salary) as total_salaries
                    from salaries
                    group by dpt_id)
select sum_sal_per_dpt.dpt_id, max(sum_sal_per_dpt.total_salaries)
from sum_sal_per_dpt
group by sum_sal_per_dpt.dpt_id
union
select sum_sal_per_dpt.dpt_id, min(sum_sal_per_dpt.total_salaries)
from sum_sal_per_dpt
group by sum_sal_per_dpt.dpt_id

有谁能解释一下吗？