SELECT Marche.PosteDepenseM, Marche.LotM, Marche.EntrepriseM, Marche.MontantM, MontantA
FROM Marche
LEFT OUTER JOIN Avenant ON Marche.Id_Marche = Avenant.IdMarche
LEFT OUTER JOIN PlusValue ON PlusValue.IdMarche = Marche.Id_Marche
GROUP BY Marche.PosteDepenseM, Marche.lotM...