Question

我想要加入4个表。

Customers
Traffic
Average
Live

我想将这些表的连接数据插入“详细信息”表。表格之间的关系如下： Traffic，Average和Live表中的每一个都有一个“cid”，它是“Customers”表的主键：

Traffic.cid = Customers.id
Average.cid = Customers.id
Live.cid = Customers.id

我写的查询在这里：

INSERT INTO Details
(
cid, Customer_Name, Router_Name,
Traffic_Received,
Average_Received,
Live_Received,
date
)     
(
SELECT Customers.id AS cid, Customers.name AS Customer_Name, Traffic.Router_Name,
Traffic.Received, 
Average.Received,
Live.Received, 
Traffic.date
FROM Customers
INNER JOIN Traffic ON Customers.id=Traffic.cid
INNER JOIN Average ON Customers.id=Average.cid
INNER JOIN Live ON Customers.id=Live.cid
WHERE Traffic.date='2015-06-08'
)

但结果会有重复的行。我将JOIN更改为LEFT JOIN和RIGHT JOIN。但结果没有改变。如何在Details表中没有重复的行？

Answer 1

你能试试吗

        INSERT INTO Details
    (
        cid, Customer_Name, Router_Name,
        Traffic_Received,
        Average_Received,
        Live_Received,
        date
    )     
    (
    SELECT Customers.id AS cid, 
        Customers.name AS Customer_Name, 
        Traffic.Router_Name,
        Traffic.Received, 
        Average.Received,
        Live.Received, 
        Traffic.date
    FROM Customers
    INNER JOIN Traffic ON Customers.id=Traffic.cid
    INNER JOIN Average ON Customers.id=Average.cid
    INNER JOIN Live ON Customers.id=Live.cid
    WHERE Traffic.date='2015-06-08'
    GROUP BY
        cid,
        Customer_Name,
        Traffic.Router_Name,
        Traffic.Received, 
        Average.Received,
        Live.Received, 
        Traffic.date
    )

Answer 2

SELECT Customers.id AS cid, Customers.name AS Customer_Name, Traffic.Router_Name,
Traffic.Received, 
Average.Received,
Live.Received, 
Traffic.date
FROM Customers
LEFT JOIN Traffic ON Customers.id=Traffic.cid
LEFT JOIN Average ON Traffic.cid=Average.cid
LEFT JOIN Live ON Average.cid=Live.cid
WHERE Traffic.date='2015-06-08'

Answer 3

使用LEFT JOIN，即使没有与Traffic对应的记录，您也会加入表格（例如Customers.id），在这种情况下，您将获得此表中没有匹配记录的列的null值。

使用RIGHT JOIN，即使Customers中没有相应的记录，您也会从联接表中获取所有记录。

但是，JOIN的类型不是问题。如果您在结果中获得重复记录，则表示您要加入的表中有多个匹配记录。例如，Traffic中可能有多条记录与cid相同。使用SELECT DISTINCT删除重复项，或者如果您对这些重复项的汇总感兴趣，请使用汇总函数，例如count()或sum()和GROUP BY子句，例如GROUP BY Traffic.cid。

如果你仍然有重复项，那么检查以确保它们确实是重复的 - 我建议一个或多个列实际上是不同的。

多个连接中的哪个JOIN类型

3 个答案: