同一张桌子上有多个JOIN

Question

I'm working on a messaging system on Laravel and I have the following tables :

USER

user_id | email | password | name | ...
---------------------------------------

CHAT

chat_id | user1_id | user2_id
-----------------------------

MESSAGE

message_id | content | date | user_id | chat_id | ...
------------------------------------------------------

and what I want to do, is to get all the chats the authenticated user have started with an other one, ordered by the date of the messages (from the most recent to the oldest one), plus the information on the user he's talking to.

I'm currently working on the raw SQL request before passing it in Laravel and this it what I try but it doesn't give the waited result :

SELECT user.user_id, user.name, user.surname
FROM user
JOIN chat a
ON user.user_id = a.user1_id
JOIN chat b
ON user.user_id = b.user2_id
WHERE b.chat_id IN
    (SELECT chat_id 
     FROM chat 
     WHERE user1_id = 1 OR user2_id = 1) 
AND user.user_id != 1  

----> (I'm testing with the user that has the ID #1)

If anyone could help that would be greatly appreciated.

Answer 1

You can use Union to get both results as follows:

select a.user1_id, a.chat_id, user.fname, user.lname 
from user
join chat a on a.user2_id = user.user_id and a.user1_id = 1
union 
select b.user2_id, b.chat_id, user.fname, user.lname 
from user
join chat b on b.user1_id = user.user_id and user2_id = 1