从三个表计算总和

Client_courses
Client_courses_id Client_id Course_id
1                    1          2
2                    1          3
3                    2          1

Client
Client_id Name
1         Tom
2         John

Courses
Course_id Name         Price
1         Microsofr    100
2         Programming  250
3         Leadership   300

SELECT SUM(c.Price), cl.Name
FROM Client cl
INNER JOIN Client_courses clc ON cl.Client_id=clc.Client_id
INNER JOIN Courses cs ON clc.Course_id=cs.Course_id
GROUP BY cl.Name