messages between users: show conversations - php

Question

I am coding a message system for user communication. In the inbox I do not want to show the user the messages he/she received. I just want to show the conversations. So as an example, if one user send or receive more than one message then in the inbox there should only be the conversation (which includes the newest message, either written or received) with the user and when the user clicks on the conversation he/she can see all past messages.
The table structure (simplified) of 'messages' is as followed:
message_id
user_id_sender
user_id_recipient
message
Now the problem is that the messages are saved in a database where each row is one message, so I have to group these messages in a certain way.
The select statement I came up with is the following:
SELECT * FROM messages
WHERE user_id_sender = 1 OR user_id_recipient = 1
GROUP BY user_id_sender
But now I obviously get two messages because one which has been written by user '1' and one that he has received..
Does anybody have an idea how to solve this?

Answer 1

I've solved this problem some month ago. I suppose you have also a date field. This query give you a well structured results with date of last message and last message.
$qry = 'SELECT
CONCAT(GREATEST(user_id_sender,user_id_recipient)," ",LEAST(user_id_sender,user_id_recipient)) AS thread,
MAX(CONCAT(date,"|",message)) as date_message,
MAX(date) AS last_message,
messages.*
FROM messages
WHERE user_id_sender= ? || user_id_recipient=? GROUP BY thread ORDER BY last_message DESC';
$rows = $db->fetchAll($qry, Array($current_user_id,$current_user_id));

Answer 2

Assuming message_id is ascending (i.e. higher ids are for later messages). #user_id is just a placeholder for the user_id of the inbox you are looking at. I've used Andrea's trick for getting the other_recipient_id concisely.
Select
mm.other_recipient_id,
m.*
From (
Select
user_id_sender + user_id_recipient - #user_id as other_recipient_id,
Max(message_id) as message_id
From
messages
Where
user_id_sender = #user_id Or
user_id_recipient = #user_id
Group By
user_id_sender + user_id_recipient - #user_id
) mm
Inner Join
messages m
On
mm.message_id = m.message_id
Example fiddle http://sqlfiddle.com/#!2/05d191/3/0

Answer 3

You get only one message, as long as the messages (message_id) are unique. You will get multiple messages, of course, if there is a longer communication going on.
N.B.: You don't need group by here, because this is used only for aggregating columns, e.g.:
SELECT user_id_sender, sum(*) FROM messages
GROUP BY user_id_sender;
where you get the sum of messages each user has sent.
So, if you want to see the communication between two users:
SELECT * FROM messages
WHERE user_id_sender = 1 OR user_id_recipient = 1;
You can restrict this further, if you store the timestamps as well, message_time for example or limit the number of messages displayed:
select * from ... limit 10;

Answer 4

Assuming we want to see conversations of user with certain _ ID _, this query can be useful:
SELECT (user_id_sender + user_id_recipient) - _ID_ AS correspondent, COUNT(*) AS total
FROM messages
WHERE user_id_sender = _ID_ OR user_id_recipient = _ID_
GROUP BY (user_id_sender + user_id_recipient)
The resulting query returns the ID of the other user of the conversation ("correspondent") and the number of messages between the two users ("total").
Regards