SELECT c.item_midclsno as item_clsno_second,

         Month(a.oper_date) oper_date,

         (select item_clsname from t_bd_item_cls where item_clsno = c.item_midclsno ) as item_clsname,   

         sum(a.sale_qty),   

         sum(a.sale_amt),   

         sum(a.ret_qty),   

         sum(a.ret_amt),   

         sum(a.giv_qty),   

         sum(a.giv_amt)

    FROM t_rm_daysum a,   

         t_bd_item_cls b,   

         (select substring(item_clsno, 1, 2*convert(int,(select sys_var_value from t_sys_system where sys_var_id = 'gl_itm_cls_nolen'))) as item_midclsno,* from t_bd_item_info) c  

   WHERE  b.item_clsno = c.item_midclsno and  

         ( a.item_no = c.item_no ) and  

         ( a.oper_date BETWEEN '2022-01-01' AND '2022-11-30' ) AND  

         ( c.item_clsno like '%' ) and

         ( a.branch_no like '%' ) and

         ( a.supcust_no like '%' ) and

         ( b.item_flag = '0' )  

GROUP BY item_midclsno,Month(a.oper_date)

肯定要把时间放到分组里面的，给时间字段加个MONTH（时间字段）格式化放在group by 后面，以月份来分组，查询的字段里也可以以加一个MONTH(时间字段) 月份

Month(a.oper_date)获取月份，再用group by按月分组

SELECT c.item_midclsno as item_clsno_second,

       month(a.oper_date) mon,

       (select item_clsname from t_bd_item_cls where item_clsno = c.item_midclsno) as item_clsname,

       sum(a.sale_qty),

       sum(a.sale_amt),

       sum(a.ret_qty),

       sum(a.ret_amt),

       sum(a.giv_qty),

       sum(a.giv_amt)

  FROM t_rm_daysum a,

       t_bd_item_cls b,

       (select substring(item_clsno,1,2 *

                         convert(int,

                                 (select sys_var_value

                                    from t_sys_system

                                   where sys_var_id = 'gl_itm_cls_nolen'))) as item_midclsno,

               *

          from t_bd_item_info) c

 WHERE b.item_clsno = c.item_midclsno

   and a.item_no = c.item_no

   and a.oper_date BETWEEN '2022-01-01' AND '2022-11-30'

   and c.item_clsno like '%'

   and a.branch_no like '%'

   and a.supcust_no like '%'

   and b.item_flag = '0'

 GROUP BY item_midclsno,month(a.oper_date)