Select SUM(IsNull(Null,0) + IsNull(NULL,0)) AS valor     -> 0
Select SUM(IsNull(Null,0)) AS valor   -> 0
Select SUM(IsNull(Null,6)) AS valor   -> 6
Select SUM(IsNull(Null,0) + IsNull(NULL,0)) AS valor     -> 0
Select SUM(IsNull(Null,0)) AS valor   -> 0
Select SUM(IsNull(Null,6)) AS valor   -> 6