requête la différence de deux somme

SELECT SUM(a.quantité)-SUM(b.quantité) AS "SOLDE" FROM livraison a,livraison b
WHERE 
    a.signe=-1; --operation achat
 AND 
    b.signe=1;  --operation vente
 AND
    a.dateOperation between x and y
 AND
    b.dateOperation between x and y

SELECT
(select sum(a.[quantite])from livraison a where a.signe=1)
-
(select sum(a.[quantite]) from livraison a where a.signe=-1) 
AS SOLDE