SELECT A.album
FROM disco AS A
LEFT JOIN hitmachine AS B ON A.id = B.id_album
WHERE A.id_groupe = 1 AND (B.del = 0 OR B.del IS NULL)
(faut pas de ' ' pour les nombres)
Sinon, un truc dans le genre, mais qui risque d'être moins performant :
SELECT A.album, IFNULL(B.del, 0) AS delete
FROM disco AS A
LEFT JOIN hitmachine AS B ON A.id = B.id_album
WHERE A.id_groupe = 1 AND delete = 0