MySQL根据最新条目对多个列进行不同选择

Question

For the inbox of my users I would like to obtain the subject of only the last message in a conversation.

As you can see the user with id 1 has 2 conversations going. One with the QA and one with the Admin. So I would like to obtain the subject of message id:4 and id:6.

I have tried to do this in a single query with DISTINCT and GROUP BY created_at without success. Both id and created_at can be used to obtain the last message in a conversation but I prefer to use created_at.

Answer 1

select 
  *,
  least   (from_user_id, to_user_id) as min_uid,
  greatest(from_user_id, to_user_id) as max_uid
from
  (select * from message order by id desc) as message_reversed
group by
  min_uid,
  max_uid

Answer 2

SELECT id
     , created_at
     , from_user_id
     , to_user_id
     , subject
     , text 
  FROM message m
  JOIN 
     ( SELECT MAX(id) max_id
         FROM message
        GROUP
           BY LEAST(from_user_id,to_user_id)
            , GREATEST(from_user_id,to_user_id)
     ) x
    ON x.max_id = m.id;

Answer 3

Given that the "id" is an auto-incrementing primary key and you know the id of the user you are working with, why don't you query like this?

SELECT `text` as `lastMessage` 
FROM `yourTable` 
WHERE `from_user_id` = 1 
ORDER BY `id` DESC 
LIMIT 0,1