Question

问题：编写查询以显示学生人数最少的部门的名称。根据部门名称升序对结果进行排序

我的代码：

select department_name
     from department d join student s
     on d.department_id=s.department_id
     having count(*) in (select min(count(student_id))
                         from student s join department d
                         on s.department_id=d.department_id
                         group by d.department_id)
group by department_name
order by department_name;

注意： Student，Department是单独的表
department_id是Department表中的主键
department_id是Student表中的ForeignKey
代码运行完美（只是需要缩短它）。

Answer 1

我会为此使用一个窗口函数：

select d.*
from (
  select s.department_id, 
         count(*) as student_count,
         dense_rank() over (order by count(*)) as rnk
  from student s 
  group by s.department_id
) as c
  join department d on d.department_id = c.department_id 
where c.rnk = 1
order by d.department_name;

如果需要，还可以在输出中包括实际的学生人数。

以上仅包括有学生的部门。如果您需要照顾没有分配学生的部门，则需要外部加入：

select d.department_name
from (
  select d.department_name,
         count(s.student_id)  as student_count, 
         dense_rank() over (order by count(s.student_id)) as rnk
  from department d 
    left join student s on s.department_id = d.department_id
  group by d.department_name
) d
where d.rnk = 1;

这两种解决方案都是100％标准的ANSI SQL，并且可以在任何现代DBMS上使用。

有什么办法可以缩短以下代码？

1 个答案: