Question

请帮助我解决这个问题。

SELECT COUNT (name.IN) AS Total, name.St_Name, name.Class_Id,
    teacher.Class_Id
  FROM name
  Left Join teacher ON  name.Class_Id = teacher.Class_Id
  WHERE name.IN = '1';

问题：我如何COUNT(name.IN)总计name.Class_Id和teacher.Class_Id相同的学生人数。

Answer 1

如果使用诸如COUNT或SUM或MAX的聚合函数？

然后，当您将其他字段添加到SELECT时，这些字段应位于GROUP BY中。

对于大多数数据库都是如此。但是MySql可能是该SQL92规则的例外。
在MariaDB中，如果使用ONLY_FULL_GROUP_BY SQL_MODE，则不允许从GROUP BY中忽略那些字段。

所以尝试一下：

SELECT 
 COUNT(name.IN) AS Total,
 COUNT(DISTINCT name.St_Name) AS TotalUniqueStudentNames, 
 COUNT(teacher.Class_id) AS TotalMatchingTeacherClass,
 name.Class_Id
FROM name
LEFT JOIN teacher ON name.Class_Id = teacher.Class_id
WHERE name.IN = '1'
GROUP BY name.Class_Id
ORDER BY name.Class_Id

Answer 2

一个SELECT语句子句，将查询结果分为几组行，通常是为了对每一组执行一个或多个聚合。 SELECT语句每组返回一行

    SELECT (name.IN) AS Total, name.St_Name, name.Class_Id, teacher.Class_Id
        FROM name
        Left Join teacher ON  name.Class_Id = teacher.Class_Id
        WHERE name.IN = '1'
group by  name.St_Name, name.Class_Id, teacher.Class_Id

依靠左联接的质素

2 个答案: