Private messaging system. Listing last message of each conversation

Question

Lets say this is the database structure:

SELECT * FROM `pms` where         


        

Answer 1

This query should work:

SELECT a.*
FROM pms a
     INNER JOIN (
                 SELECT id_to, id_from, MAX(fecha) AS fecha
                 FROM pms
                 WHERE (id_to = 1 OR id_from = 1)
                 GROUP BY LEAST(id_to, id_from)
                ) b
                ON a.fecha = b.fecha AND
                   (a.id_to = b.id_to OR
                   a.id_from = b.id_from);

See example @ sqlfiddle here

If you have id as PRIMARY KEY and you are logging messages in a chronological order, then it can be further optimized and simplified as:

SELECT a.*
FROM pms a
     INNER JOIN (
                 SELECT MAX(id) AS id
                 FROM pms
                 WHERE (id_to = 1 OR id_from = 1)
                 GROUP BY LEAST(id_to, id_from)
                ) b
                ON a.id = b.id;