mysql - 用不同的表做一些相当基本的求和

Question

我有下表:

交易:Transaction_ID Datetime Giver_ID Recipient_ID Points Category_ID Reason

奖励:Reward_ID Title Description Image_URL Date_Inactive Stock_Count Cost_to_User Cost_to_System

购买:Purchase_ID Datetime Reward_ID Quantity Student_ID Student_Name Date_DealtWith Date_Collected

<小时/>

工作人员向学生奖励积分，这些积分将成为交易表的条目，如下所示:

<强> Transaction_ID Datetime Giver_ID Recipient_ID Points Category_ID Reason
8 2011-09-07 36761 127963 2 1 Excellent behaviour in behaviour unit

然后，学生可以购买奖励，该奖励将成为购买表的一个条目，如下所示:

<强> Purchase_ID Datetime Reward_ID Quantity Student_ID Student_Name Date_DealtWith Date_Collected
1570 2012-05-30 12:46:36 2 1 137616 Bradley Richardson NULL NULL

所有奖励均手动存储在奖励数据库表中:

<强> Reward_ID Title Description Image_URL Date_Inactive Stock_Count Cost_to_User Cost_to_System
1 Lunch Time Queue Pass (month) Beat the queue and get to the hot food early! /user/74/167976.png 2012-04-16 11:50:00 0 100 0

<小时/>

我的问题是这样的:
给定 Reward_ID 和 Student_ID，我可以使用什么 SQL 语句来计算学生是否有足够的积分来购买奖励？

我之前曾在 PHP 中完成过此操作，但如果可能的话，我希望能够使用单个 SQL 语句来完成此操作。伪代码类似于 SELECT ( SumOfPointsEarned - SumOfPointsOfPurchasesMade )但我完全不知道该怎么做!

如有任何建议，我们将不胜感激。

Answer 1

一种方法是使用内联 View 。为此，

• 创建一个查询来汇总所获得的积分
• 创建一个查询，对所花费的积分进行求和
• 将这些查询连接在一起(左连接，以防它们尚未花费任何内容)
• 做减法

SELECT SumOfPointsEarned.Points - COALESCE(SumOfPointsOfPurchasesMade.Points, 0) AS CurrentPoints
FROM   
(
    SELECT SUM(Points) AS Points, Recipient_ID 
    FROM   transactions 
            WHERE Recipient_ID= 137642
    GROUP  BY Recipient_ID
) AS SumOfPointsEarned 
    LEFT JOIN 
    (
        SELECT purchases.Student_ID,  SUM(rewards.Cost_to_User) AS Points 
        FROM   purchases 
            INNER JOIN rewards 
            ON purchases.Reward_ID = rewards.Reward_ID 
                    WHERE Student_ID = 137642
        GROUP  BY purchases.Student_ID
    ) AS SumOfPointsOfPurchasesMade 
    ON SumOfPointsEarned.Recipient_ID = SumOfPointsOfPurchasesMade.Student_ID