Question

Lets say this is the database structure:

SELECT * FROM `pms` where id_to = 1 or id_from = 1

This would return all the messages that he has recived or sent,

So how can I retrieve the last message from each conversation that the user 1 may have?

PD: I call it conversation when there is one or more messages between two users

-edit-

So given this database content:

We want to get id 4 and 6

Answer 1

This assumes id is an auto-increment column:

SELECT MAX(id) AS id
FROM pms
WHERE id_to = 1 OR id_from = 1
GROUP BY (IF(id_to = 1, id_from, id_to))

Assuming you have id_from and id_to indexed, this variation will most likely perform better because MySQL doesn’t know what to do with an OR:

SELECT MAX(id) AS id FROM
(SELECT id, id_from AS id_with
FROM pms
WHERE id_to = 1
UNION ALL
SELECT id, id_to AS id_with
FROM pms
WHERE id_from = 1) t
GROUP BY id_with

Here’s how to get the messages for those ids:

SELECT * FROM pms WHERE id IN
    (SELECT MAX(id) AS id FROM
    (SELECT id, id_from AS id_with
    FROM pms
    WHERE id_to = 1
    UNION ALL
    SELECT id, id_to AS id_with
    FROM pms
    WHERE id_from = 1) t
    GROUP BY id_with)