SELECT MOQ.MONTH, AVG(MOQ.SUM) "Average"
FROM
    (SELECT MONTH(S_SOH.OrderDate) "Month", SUM(S_SOD_OQ.OQ) "Sum"
    FROM Sales.SalesOrderHeader S_SOH
        JOIN (SELECT S_SOD.SalesOrderID, SUM(OrderQty) "OQ" FROM Sales.SalesOrderDetail S_SOD GROUP BY S_SOD.SalesOrderID) S_SOD_OQ
            ON S_SOH.SalesOrderID = S_SOD_OQ.SalesOrderID
    GROUP BY MONTH(S_SOH.OrderDate), YEAR(S_SOH.OrderDate)) "MOQ"
GROUP BY MOQ.MONTH;