如何在MySQL联接查询中使用IF语句? [英] How do I use an IF statement in an MySQL join query?

问题描述

我有一个SQL查询，该查询根据条件以不同的方式连接表.

I have a SQL query that left joins a table in different ways depending on a condition.

SELECT m.id, u.first_name AS otherUser
FROM matches AS m
IF (u.id=m.user2ID)
    LEFT JOIN users AS u ON u.id = m.user1ID
ELSE
    LEFT JOIN users AS u ON u.id = m.user2ID
ENDIF
WHERE m.user1ID=2 OR m.user2ID=2

matches是具有整数列user1ID和user2ID的表. users是一个包含我的Web应用程序用户的表. users有一个名为first_name的VARCHAR字段.

matches is a table with integer columns user1ID and user2ID. users is a table containing users of my web application. users has a VARCHAR field called first_name.

此查询的目的是获取与当前用户匹配的用户名.

The intention of this query is to get the names of the users matched with the current user.

但是，MySQL返回此错误:

However, MySQL returns this error:

#1064 - You have an error in your SQL syntax; 
check the manual that corresponds to your MySQL server version for 
the right syntax to use near 'IF (u.id=m.user2ID) LEFT JOIN users 
AS u ON u.id = m.user1ID ELSE LEFT JOIN user' at line 3

为什么?

推荐答案

将条件指定为ON子句的一部分:

Specify the condition as part of the ON clause:

SELECT m.id, u.first_name AS otherUser
FROM matches AS m
LEFT JOIN users AS u ON (u.id=m.user2ID and u.id = m.user1ID) or (u.id<>m.user2ID and u.id = m.user2ID) 
WHERE m.user1ID=2 OR m.user2ID=2

做同样事情的另一种方法:

Another way to do the same thing:

SELECT m.id, u.first_name AS otherUser
FROM matches AS m
LEFT JOIN users AS u ON IF(u.id=m.user2ID,u.id = m.user1ID,u.id = m.user2ID) 
WHERE m.user1ID=2 OR m.user2ID=2

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