如何从两个表中选择和计算数据？

I have a table of facture and a table of fournisseur, but I want to select all my fournisseur from the database and I want to calculate the field nvsolde (solde-SUM(facture.montant)). But when I use this request it gives just fournisseur already had a facture.I have a table of facture and a table of fourni