私人聊天系统-获取每次聊天的最新消息

Question

I am making a chat system between 2 users. I have chat table that looks like that: (Here is the schema: SQL Fiddle )

        --------------------------------------------------------------------------------
        | message_id | sender_id | receiver_id  | message_text  |       date           |
        --------------------------------------------------------------------------------
        |     1      |     20    |      100     |  Hello mate   |  2018-04-29 12:15:33 | 
        --------------------------------------------------------------------------------
        |     2      |    100    |      20      |   Hi there    |  2018-04-29 13:20:05 | 
        --------------------------------------------------------------------------------
        |     3      |     13    |      44      |   Alright     |  2018-04-29 14:35:15 | 
        --------------------------------------------------------------------------------
        |     4      |     20    |      57      |   Hi user 57  |  2018-04-29 16:44:34 | 
        --------------------------------------------------------------------------------

I want to get the rows of the last messages from each chat a user is engaged in, so in the above example, user 20 should get 2 rows: row of message_id 2 (Because he is engaged in a chat with user 100, although he did not send the last message), and row of message_id 4 (because he sent message to user 57) It's basically like WhatsApp where you always have the first messages of any chat at the top.

The problem is I can only get the last message if the user is the SENDER and not if he is the receiver:

 SELECT * FROM chat 
 WHERE message_id IN 
(SELECT MAX(message_id) FROM chat WHERE sender_id=:sender_id 
 GROUP BY receiver_id) ORDER BY date DESC

I think I should add another column "chat_id" which would be a number that is identical for each 2 users that are in the same chat (so user 100 and 20 will have same chat_id whether they are the senders or receivers, and users 13 and 44 will have a different in this example),

Or, there is a better option?

Answer 1

You could use an union between sender and receiver for get a result for both

  You could use 
  select * 
  from chat 
  inner join  (
    select max(message_id) max_id, user from (
    select message_id, sender_id as  user
    from chat
    union 
    select message_id, receiver_id 
    from chat) t 
    ) t1 on t1.max_id = chat.message_id
          and chat.sender_id=:sender_id 

Answer 2

To handle this efficiently, start with these two queries:

select message_id, sender_id as other_id
from messages
where receiver_id = :user
order by message_id desc;

select message_id, receiver_id as other_id
from messages
where sender_id = :user
order by message_id desc;

These can each be optimized using two indexes: messages(receiver_id, sender_id, message_id) and messages(sender_id, receiver_id, message_id) .

You can union all them together and sort or aggregate to get the maximum value:

select m.*
from messages m
where m.message_id in (select max(message_id)
                       from ((select message_id, sender_id as other_id
                              from messages
                              where receiver_id = :user
                              order by message_id desc
                              limit 1
                             ) union all   
                             (select message_id, receiver_id as other_id
                              from messages
                              where sender_id = :user
                              order by message_id desc
                              limit 1
                             )
                            ) mm
                      );

The subqueries should be able to make use of the indexes (actually, the "other id" is not needed). The max() is then on two rows, and the in should be able to take advantage of an index on (message_id) .

EDIT:

For the revised problem, I can't think right of hand of a really efficient method. Here is one method:

select m.*
from messages m
where m.message_id in (select max(m2.message_id)
                       from messages m2
                       where :user in (sender_id, receiver_id)
                       group by (case when sender_id = :user then receiver_id else sender_id end)
                      );