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是否可以根据两列的组合执行计数？

我有 3 张桌子：

学生： Student id | Student name | Gender | Dob |

班级登记：Student id | Class id

两列构成复合主键。

课程：Class id | Class name | Class day

我想计算每个班级中每种性别的学生人数。

到目前为止，我想出了这个查询：

SELECT class_name AS 'class Name',
  Child_gender AS 'Gender',
  COUNT(student_id) AS 'Count'
FROM student,classRegister,classes
WHERE student.student_id=classRegister.student_id AND
  classRegister.class_id=classes.class_id
GROUP BY class_name,student_gender
ORDER BY class_name;

但它似乎给了我一些学生重复的结果。

那么，我可以修改此查询以计算 和 组合的实例数student_id吗class_id？

我对 MySQL 还是很陌生，所以如果这对每个人来说都是基本的，我很抱歉

4

2 回答 2

1

试试这个：

SELECT c.class_name AS 'class Name', s.Child_gender AS 'Gender',
       COUNT(s.student_id) AS 'Count'
FROM student s
INNER JOIN classRegister cr ON s.student_id = cr.student_id 
INNER JOIN classes c ON cr.class_id = c.class_id
GROUP BY s.Child_gender, s.class_id, c.class_name
ORDER BY c.class_name
于 2012-04-30T15:00:48.537 回答
0 
SELECT c.class_name AS 'class Name', s.Child_gender AS 'Gender',
       COUNT(DISTINCT s.student_id) AS 'Count'
FROM student s
INNER JOIN classRegister cr ON s.student_id = cr.student_id 
INNER JOIN classes c ON cr.class_id = c.class_id
GROUP BY c.class_name, s.Child_gender

如果他们在 classRegister 中有多个注册，您可能会多次获得同一个学生。这听起来很恶心。COUNT 中的 DISTINCT 应​​确保每个班级只计算一次。

于 2012-04-30T15:09:36.567 回答