MySQL查询某类的书本费用 [英] MySQL Query for book cost of a class

问题描述

对于以下内容，我只需要一个MySQL行查询即可.

I just need a single MySQL line query for the following.

让我们说有2个简单的表: Class 和 Books

Lets say there are 2 simple tables: Class and Books

Class
ID--Name-----Students
1 ------公斤----------- 20
2 ------等级(3)--- 25
3 ------等级(5)--- 30

Class
ID--Name-----Students
1------KG-----------20
2------Grade(3)---25
3------Grade(5)---30

Books
ID--ClassId--Title-------------Cost
1 ----- 1 --------------绘图------------------ 5
2 ----- 3 --------------历史-------------------- 25
3 ----- 1 -------------- A-to-Z -------------------- 10
4 ----- 2 --------------字母----------------- 20
5 ----- 3 --------------数学-------------------- 15
6 ----- 2 --------------英语------------------- 30

Books
ID--ClassId--Title-------------Cost
1-----1--------------Drawing------------------5
2-----3--------------History--------------------25
3-----1--------------A-to-Z--------------------10
4-----2--------------Alphabets---------------20
5-----3--------------Maths--------------------15
6-----2--------------English-------------------30

让我们说:
我们只知道 Class
的-----> ID 我们要找到的是---> 一堂课的书本费用.(每班学生的书本)

Lets say:
What we only know is -----> ID of the Class
What we have to find is ---> Book Cost of a class. (Books for Each Students In A Class)

我可以只查询一行吗?

推荐答案

尝试一下:

单行:

SELECT SUM(Class.Students * Books.Cost) AS BookCost  FROM Books INNER JOIN Class       ON Books.ClassId = Class.ClassId WHERE Books.ClassId = <CLASS-ID-VALUE>  GROUP BY Books.ClassId

使用格式:

SELECT SUM(Class.Students * Books.Cost) AS BookCost
  FROM Books INNER JOIN Class
   ON Books.ClassId = Class.ClassId
WHERE Books.ClassId = <CLASS-ID-VALUE>
GROUP BY Books.ClassId

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