My update function does not always work properly, especially when one of the fields is 0 or blank.How could i improve my function so that when addding wih 0 not to give 0 ?

My function is the following:
Public Function dummy()
StrSQL = " UPDATE (products1 INNER JOIN products ON products1.Productid = products.Productid) INNER JOIN [Order Details1] ON " & _


" products1.Productid = [Order Details1].productid SET products1.items1 = [order details1].[quantity]+[products1].[items1]"
End Function