Privat besked system. Viser sidste besked for hver samtale

Dette forudsætter id er en kolonne med automatisk stigning:

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))

Forudsat at du har id_from og id_to indekseret, vil denne variant højst sandsynligt fungere bedre, fordi MySQL ikke ved, hvad de skal gøre med en 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

Sådan får du beskederne til disse id'er:

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)