sql 联表排序有关问题

sql 联表排序问题
MF表
PS_NO(订单号)     CUS_NO(客户代码)  
    1                                张三
    2                                李四

    3                                张三

    4                                陈五

TF表
PS_NO(订单号)      QTY(出货数量)      AMTN(出货金额)
    1                                10                            1000

    2                                30                            12000

    3                                2                               200

    4                                21                            4000

需求：如何根据客户总出货量进行排序
CUS_NO(客户代码)      QTY(出货数量)         AMTN(出货金额)
     李四                                30                            12000
     陈五                                21                            4000
     张三                                12                            1200
------解决思路----------------------
--加上排序倒序

SELECT a.CUS_NO,SUM(Qty) AS QTY,SUM(AMTN) AS AMTN FROM MF AS a INNER JOIN TF AS b ON a.PS_NO=b.PS_NO GROUP BY a.CUS_NO

ORDER BY 2 DESC,3 DESC