CREATE TABLE forum_lu (
  id int(11) NOT NULL AUTO_INCREMENT, --pas obligatoire
  auteur_id int(11) NOT NULL DEFAULT '0',
  sujet_id int(11) NOT NULL DEFAULT '0',
  dernier_id int(11) NOT NULL DEFAULT '0',
  nb int(11) NOT NULL DEFAULT '0', --utile pour créer un lien direct
  PRIMARY KEY  (id)
);

SELECT s.id, s.titre, s.sous_titre, s.auteur_id, s.etat, s.nb_rep, s.nb_lu, m.pseudo AS s_pseudo, lu.dernier_id AS dernier_lu, lu.nb, s.dernier, DATE_FORMAT(d.auteur_date, "%d/%m/%Y %H:%i") d_date, md.id AS d_id, md.pseudo AS d_pseudo, UNIX_TIMESTAMP(d.auteur_date) AS s_date
FROM forum_sujet AS s
LEFT JOIN membres AS m ON s.auteur_id = m.id
LEFT JOIN forum_message AS d ON s.dernier = d.id
LEFT JOIN membres AS md ON d.auteur_id = md.id
LEFT JOIN forum_lu AS lu ON s.id = lu.sujet_id AND lu.auteur_id='.$sess_id.' --ici
WHERE s.id_forum='.$id.' AND s.etat&2
ORDER BY d.auteur_date DESC

SELECT * FROM machin WHERE truc IN (SELECT truc FROM bidule WHERE plop='Hihi');

SELECT * FROM
    (SELECT nom, prenom FROM membres LIMIT 10)
WHERE age>18