Question

假设我有三个表：Customer，TV，Radio

Customer表包含以下属性：customerID，First name，Last name

TV表包含以下属性：customerID，year，numTVsBought

Radio表包含属性，customerID，year，numRadiosBought

因此，在电视和广播中，特定的客户ID可以多次出现，对应不同的年份。

现在我想获取一个包含CustomerID，TotalTVsBought，TotalRadiosBought的表

当我这样做时：

SELECT CustomerID, SUM(numTVsBought), SUM(numRadiosBought)
FROM Customer
LEFT JOIN TV on Customer.customerID = TV.customerID LEFT JOIN Radio on Customer.customerID = Radio.customerID
GROUP BY Customer.customerID;

我为numTVsBought获得了正确的值，但是numRadiosBought被夸大了。我是以错误的方式加入表格吗？

Answer 1

是的，您加入表格的方式是，对于每台电视，您也会添加无线电。试试这个：

SELECT c.customerId, ifnull(tv_count.counter, 0) as totalTvsBought, ifnull(radio_count.counter,0) as totalRadiosBought
FROM customer c
LEFT OUTER JOIN (
    SELECT customerId, SUM(*) as counter
    FROM TV 
    GROUP BY customerId) tv_count
        ON tv_count.customerId = c.customerId
LEFT OUTER JOIN (
    SELECT customerId, SUM(*) as counter
    FROM Radio 
    GROUP BY customerId) radio_count
        ON radio_count.customerId = c.customerId

连接多个表时，您需要像计算机一样思考;在计算之前他会加入所有记录。

Answer 2

在SQL Server中，我们可以使用公用表表达式（CTE）

来实现它

WITH TBL1 AS 
(
        SELECT C.CustID AS CUST_ID, SUM(T.NoOfTVsBought) AS TOTAL_T From Customer C
        LEFT JOIN TVs T
        ON C.CustID = T.CustID
        GROUP BY C.CustID
)
SELECT CUST_ID, TOTAL_T, TBL2.TOTAL_Radios FROM TBL1
LEFT JOIN(
SELECT C.CustID, SUM(R.NoOfRadBought) AS TOTAL_Radios From Customer C
LEFT JOIN Radios R
ON C.CustID = R.CustID
GROUP BY C.CustID) AS TBL2
ON TBL1.CUST_ID = TBL2.CustID

如何制定MYSQL查询

2 个答案: