Question

我有如下的sql：

select a.dept, a.name
  from students a
 group by dept, name
 order by dept, name

得到结果：

dept   name
-----+---------
CS   | Aarthi
CS   | Hansan
EE   | S.F
EE   | Nikke2

我想总结一下每个部门的学生人数如下：

dept   name        count
-----+-----------+------  
CS   | Aarthi    |  2
CS   | Hansan    |  2
EE   | S.F       |  2
EE   | Nikke2    |  2
Math | Joel      |  1

我该怎么写sql？

Answer 1

虽然看起来你没有显示所有表格，但我只能假设每个学生还有另一张实际入学表格

select a.Dept, count(*) as TotalStudents
  from students a
  group by a.Dept

如果你想要与每个学生相关的每个部门的总数（没有意义），你可能必须这样做......

select a.Dept, a.Name, b.TotalStudents
    from students a,
        ( select Dept, count(*) TotalStudents
             from students
             group by Dept ) b
    where a.Dept = b.Dept

我对你的“姓名”栏目的解释是学生的名字而不是班级的实际讲师，因此我的子选择/加入。否则，就像其他人一样，只需使用COUNT（*）作为第三列即可。

Answer 2

select a.dept, a.name,
       (SELECT count(*)
          FROM students
         WHERE dept = a.dept)
  from students a
 group by dept, name
 order by dept, name

这是一个有点可疑的查询，因为您获得了部门计数的重复副本。获取学生列表更加清晰，部门计算为单独的结果。当然，出于另一种方式可能有实际的理由，所以这不是一个绝对的规则。

Answer 3

SELECT dept, name, COUNT(name) as CT from students
group by dept, name
order by dept, name

Answer 4

这应该这样做（我没有任何环境可以在最短时间内进行测试）

select a.dept, a.name, count(a.*) as NumOfStudents
from students a
group by dept, name order by dept, name

HTH

Answer 5

或者简单地写一下

select dept, name, count(name) as nostud from students group by dept, name order by dept, name

Answer 6

这将给出上面要求的结果

select a.dept, a.name, cnt
from student a
join (
select dept, count(1) as cnt
from student
group by dept
) b on b.dept = a.dept

计数列分组

6 个答案: