GROUP BY two fields in mysql

Question

I have a table in mysql where it stores the messages between users. The Table has the following fields:

ID   from_memberID  to_memberID   message                 Date
+----+--------------+------------+-----------------------+-----------------------+
1         205          207         hello Peter           19/08/2012 20:00:00
2         207          205         Hi frank              19/08/2012 20:01:00
3         205          208         hello Jennifer        19/08/2012 20:10:00
4         207          210         hello Peter           19/08/2012 20:20:00

Well, let's suppose I want to get only last message from each conversation by user 205

it should get registers 2 and 3, the conversation between users(205-207) and users (205-208). The fact is that i use GROUP BY from_memberID and then in my result the register ID=1 is added too.

This is the statement is use:

SELECT *
FROM
(SELECT msg.ID,
    msg.mensaje,
    msg.from_miembroID,
    msg.fecha,
    msg.read,
    FROM mensajes as msg
    INNER JOIN miembros as mem ON mem.ID=IF(msg.from_miembroID=205, msg.to_miembroID, msg.from_miembroID)
WHERE msg.to_miembroID=205 OR msg.from_miembroID=205
ORDER BY msg.fecha DESC) as temp
GROUP BY from_miembroID

how can i use GROUP BY for considering the two columns (from_memberID, to_memberID) to be as one?

Thanks in advance.

Answer 1

You could use a new custom field, ordering the ids (from_memberID, to_memberID) by a custom rule, e.g. the smaller one always comes first.

For example:

SELECT 
     IF(from_memberID < to_memberID, 
         CONCAT(from_memberID, '-', to_memberID),
         CONCAT(to_memberID, '-', from_memberID)
     ) as conversation_ids
FROM `messages`

That field will return unique combinations, e.g. 205-207 no matter if 205 sent a message to 207 or 207 sent a message to 207.

Finally, you could group by this value and get unique combinations.