MySQL：连接三个表，比较两个值

Question

首先，我是 SQL 的业余爱好者。 这是例子。 从这三张表我想知道谁是比迈克更赚钱的老师

表格1：

LessonName        TeacherID
Maths              3
Biology            2
Biology            4
Geology            1

表2：

Lesson    PricePerClass
Maths         200
Biology       100
Geology       150

表3：

IDTeacher  TeacherName
1          Mike
2          John
3          Lauren
4          Julian

到目前为止，我已经做到了：

select t3.IDTeacher, sum(t2.PricePerClass) TotalRevenue
from Table3 t3 
inner join Table1 as t1 on t1.TeacherId = t3.IDTeacher
inner join Table2 as t2 on t2.Lesson = t1.LessonName
group by t3.IDTeacher 
where TotalRevenue > (select TotalRevenue
                     from Table2 as t2
                     inner join Table1 as t1 on t2.Lesson = t3.LessonName
                     inner join Table3 as t3 on t3.IDTeacher = t1.TeacherID
                     where t3.TeacherName = "Mike");

而且我不知道如何继续，因为当我运行它时，会出现错误。

我的预期结果是这样的：

IDTeacher   TotalRevenue
3           200

谢谢！

Answer 1

在您的主查询中，您必须使用 HAVING 子句而不是 WHERE 子句，并且在子查询中修复您的连接：

select t3.IDTeacher, sum(t2.PricePerClass) TotalRevenue
from Table3 t3 
inner join Table1 as t1 on t1.TeacherId = t3.IDTeacher
inner join Table2 as t2 on t2.Lesson = t1.LessonName
group by t3.IDTeacher 
having TotalRevenue > (
                        select sum(t2.PricePerClass)
                        from Table3 t3 
                        inner join Table1 as t1 on t1.TeacherId = t3.IDTeacher
                        inner join Table2 as t2 on t2.Lesson = t1.LessonName
                        where t3.TeacherName = "Mike"
                      );

请参阅演示。
结果：

| IDTeacher | TotalRevenue |
| --------- | ------------ |
| 3         | 200          |