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.user2I         


        

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