I can't save my view when I use this query:

```
SET DATEFIRST 6

SELECT       
    ISNULL ((SELECT SUM(ISNULL(Qte_achat * Prix_unit, 0) + 
                        ISNULL(qte_achat2 * prix_unit2, 0)) AS SUM_Achat
             FROM dbo.Achat
             WHERE (DATEPART(week, date_achat) = DATEPART(week, GETDATE())) 
               AND (YEAR(date_achat) = YEAR(GETDATE())) 
               AND (used_by_vent = 'false')), 0) AS SUM_Achat 

```