困难的SQL JOIN LEFT [英] Difficult SQL JOIN LEFT

问题描述

您好！关于困难的LEFT JOIN，我有一个问题。

解释：

有3张桌子：



< pre lang =SQL> USERS
_______
Id_USER
名称
姓氏

评论
________
Id_COMMENT
Id_USER
文字

订单
________
Id_ORDER
名称
Id_USER

******************

我查询：

  SELECT  USERS.Name，USERS.Surname，COMMENTS.Id_COMMENT，COMMENTS。 text ，ORDERS.name  FROM  USERS  LEFT   JOIN  COMMENTS  ON  
 COMMENTS.Id_USER = USERS.Id_USER  LEFT   JOIN  ORDERS  ON  ORDERS.Id_USER = USERS.Id_USER   <跨度cl ass =code-keyword> LEFT   JOIN  USERS  ON  ORDERS.Id_USER = USERS。 Id_USER   

所以，我不知道如果我使用表USERS再次JOENT LEFT是多么正确想要将最后一张表ORDERS与USERS连接，如果我想从用户那里选择信息，谁在桌上订购ORDERS？



谢谢！

解决方案

  SELECT  USERS.Name，USERS.Surname，COMMENTS.Id_COMMENT，COMMENTS。 text ，ORDERS.name 
  FROM  USERS 
  LEFT   JOIN 评论 ON  COMMENTS.Id_USER = USERS.Id_USER 
  LEFT   JOIN  ORDERS  ON  ORDERS.Id_USER = USERS.Id_USER 

订单表中只列出一个用户ID，因此您只需要一个指向用户表的链接。

Hello! I have question for you about difficult LEFT JOIN.
Explaining:
There are 3 tables:

USERS
_______
Id_USER
Name
Surname

COMMENTS
________
Id_COMMENT
Id_USER
Text

ORDERS
________
Id_ORDER
Name
Id_USER

******************

I make query:

SELECT USERS.Name, USERS.Surname, COMMENTS.Id_COMMENT, COMMENTS.text, ORDERS.name FROM USERS LEFT JOIN COMMENTS ON
COMMENTS.Id_USER = USERS.Id_USER LEFT JOIN ORDERS ON ORDERS.Id_USER = USERS.Id_USER LEFT JOIN USERS ON ORDERS.Id_USER = USERS.Id_USER

So, i dont know how is correct to make JOIN LEFT again with table USERS if I want connect last table ORDERS with USERS, if i want select information from users, who made order at table ORDERS?

Thank you!

解决方案

SELECT USERS.Name, USERS.Surname, COMMENTS.Id_COMMENT, COMMENTS.text, ORDERS.name 
FROM USERS 
LEFT JOIN COMMENTS ON COMMENTS.Id_USER = USERS.Id_USER 
LEFT JOIN ORDERS ON ORDERS.Id_USER = USERS.Id_USER

You only have one User ID listed in the Order table, so you only need one link to the user table.

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