How do I use an IF statement in an MySQL join query?

Question

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 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.

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

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

Why?

Answer 1

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

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

Answer 2

Use this query:

SELECT m.id, u.first_name AS otherUser FROM matches AS m LEFT JOIN users AS u ON u.id = m.user1ID AND u.id=m.user2ID LEFT JOIN users AS u1 ON u1.id = m.user2ID WHERE m.user1ID=2 OR m.user2ID=2

Updated query:

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

Check this Source.

Also check this MYSQL Inner Join if statement . It might be helpful for you.