Question

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 |

班级注册：Student id | Class id

两列都构成复合主键。

课程：Class id | Class name | Class day

我想计算每个班级每个性别的学生人数。

到目前为止，我想出了这个问题：

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的重复结果。

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

我还是MySQL的新手，所以如果这对每个人来说都是基本的，我会道歉

Answer 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

Answer 2

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，您可能会多次同一个学生。听起来很糟糕。 COUNT中的DISTINCT应该确保每个类只计算一次。

COUNT使用两个（或更多）列

2 个答案: