mysql - 多个连接中的哪个 JOIN 类型

Question

我有 4 个要加入的表。

1. Customers
2. Traffic
3. Average
4. Live

我想将这些表的连接数据插入到“详细信息”表中。表之间的关系在这里：每个 Traffic、Average 和 Live 表都有一个“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'
)

但结果将有重复的行。我将 和 更改JOIN为LEFT JOIN和RIGHT JOIN。但结果没有改变。我应该怎么做才能在表中没有重复的行Details？

Answer 1

你能试试这个吗

        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，即使没有与 对应的记录LEFT JOIN，您也将加入表（例如） ，在这种情况下，您将从该表中获得没有匹配记录的列的值。TrafficCustomers.idnull

使用RIGHT JOIN，您将从联接表中获取每条记录，即使 中没有相应的记录Customers。

但是，JOIN这里的类型不是问题。如果您的结果中有重复记录，则这意味着您要加入的表中有多个匹配记录。例如，可能有多个记录Traffic具有相同的cid. 用于SELECT DISTINCT删除重复项，或者如果您对这些重复项的聚合感兴趣，请使用聚合函数，例如count()orsum()和GROUP BY子句，例如GROUP BY Traffic.cid.

如果您仍然有重复，请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。