SELECT Year, OrdersNum AS Y_Orders, 
            SUM(OrdersNum) OVER (ORDER BY Year) AS Running_Y_Orders
FROM (
            SELECT calendaryear AS Year, 
                        COUNT(DISTINCT orderkey) AS OrdersNum
            FROM factonlinesales fos INNER JOIN dimdate dd
                ON fos.orderdatekey = dd.datekey
            GROUP BY calendaryear
            )
ORDER BY 1, 2 ;