select
  a.地区,a.销售数量,b.退货数量,cast(b.退货数量*1.0/a.销售数量 as dec(18,2))+'%'
from
  (select 地区,订单号,sum(销售数量) as 销售数量  from a group by 地区,退货数量）a
left join
  (select 订单号,sum(退货数量) as 退货数量 from b group by 退货数量)b
on
  a.订单号=b.订单号

------解决方案--------------------
SQL code

declare @t table 
( [地区] nvarchar(4),
  [日期] datetime,
  [订单号] varchar(8),
  [销售数量] int
 
)
insert into @t
select '广东','2012-7-1','1111',2 union all
select '广东','2012-7-2','1112',1 union all
select '云南','2012-6-5','1001',1

declare @t2 table 
([订单号]    varchar(8)     ,
 [退货数量]    int
)

insert into @t2
select '1111',1union all
select '1112',1


;with t3 as 
(
    select [地区],[销售数量],[退货数量] from @t a left join @t2 b on a.[订单号] = b.[订单号]
)

select [地区],sum([销售数量]) as [销售数量],sum([退货数量]) as [退货数量], 
        sum([退货数量])*100.0/nullif(sum([销售数量]),0) as [退货率]
from t3
group by [地区]

(3 行受影响)

(2 行受影响)
地区   销售数量        退货数量        退货率
---- ----------- ----------- ---------------------------------------
广东   3           2           66.666666666666
云南   1           NULL        NULL
警告: 聚合或其他 SET 操作消除了空值。

(2 行受影响)

------解决方案--------------------
SQL code
--> 测试数据: #A
if object_id('tempdb.dbo.#A') is not null drop table #A
create table #A ([地区] varchar(4),[日期] datetime,[订单号] int,[销售数量] int)
insert into #A
select '广东','2012-7-1',1111,2 union all
select '广东','2012-7-2',1112,1 union all
select '云南','2012-6-5',1001,1

if object_id('tempdb.dbo.#B') is not null drop table #B
create table #B ([订单号] int,[退货数量] int)
insert into #B
select 1111,1 union all
select 1112,1

;with temp as(
select a.地区,SUM(b.退货数量) 退货数量,SUM(a.销售数量) 销售数量 from #A a
left join #B b on a.订单号=b.订单号
group by a.地区)
select 地区,
销售数量,
isnull(退货数量,0)退货数量,
isnull(cast(cast((退货数量+.0)/(销售数量+.0)*100 as decimal(12,2)) as varchar(10))+'%',0) 退货率 
from temp

/*
地区   销售数量        退货数量        退货率
---- ----------- ----------- -----------
广东   3           2           66.67%
云南   1           0           0
*/