需要union

一个SQL 面试题
去年应聘一个职位未果,其间被考了一个看似简单的题,但我没有找到好的大案.
不知各位大虾有无好的解法?
题为:
有两个表, t1, t2,
Table t1:
SELLER | NON_SELLER
----- -----
A    B
A     C
A     D
B     A
B     C
B     D
C     A
C     B
C     D
D     A
D     B
D     C
Table t2:
SELLER | COUPON | BAL
----- --------- ---------
A     9     100
B     9     200
C     9     300
D     9     400
A     9.5     100
B     9.5     20
A     10     80
要求用SELECT 语句列出如下结果:------如A的SUM(BAL)为B,C,D的和,B的SUM(BAL)为A,C,D的和.......
且用的方法不要增加数据库负担,如用临时表等.
NON-SELLER| COUPON | SUM(BAL)
------- --------
A     9     900
B     9     800
C     9     700
D     9     600
A     9.5     20
B     9.5     100
C     9.5     120
D     9.5     120
A     10     0
B     10     80
C     10     80
D     10     80

题解：目的是要统计NON_SELLER的信息，题目的动机我不知道不过答案倒有。

/****** Object:  Table [dbo].[t2]    Script Date: 03/15/2011 09:37:19 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[t2](
	[SELLER] [nvarchar](50) NULL,
	[COUPON] [float] NULL,
	[BAL] [int] NULL
) ON [PRIMARY]
GO
INSERT [dbo].[t2] ([SELLER], [COUPON], [BAL]) VALUES (N'A', 9, 100)
INSERT [dbo].[t2] ([SELLER], [COUPON], [BAL]) VALUES (N'B', 9, 200)
INSERT [dbo].[t2] ([SELLER], [COUPON], [BAL]) VALUES (N'C', 9, 300)
INSERT [dbo].[t2] ([SELLER], [COUPON], [BAL]) VALUES (N'D', 9, 400)
INSERT [dbo].[t2] ([SELLER], [COUPON], [BAL]) VALUES (N'A', 9.5, 100)
INSERT [dbo].[t2] ([SELLER], [COUPON], [BAL]) VALUES (N'B', 9.5, 20)
INSERT [dbo].[t2] ([SELLER], [COUPON], [BAL]) VALUES (N'A', 10, 80)
/****** Object:  Table [dbo].[t1]    Script Date: 03/15/2011 09:37:19 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[t1](
	[SELLER] [nvarchar](50) NULL,
	[NON_SELLER] [nvarchar](50) NULL
) ON [PRIMARY]
GO
INSERT [dbo].[t1] ([SELLER], [NON_SELLER]) VALUES (N'A', N'B')
INSERT [dbo].[t1] ([SELLER], [NON_SELLER]) VALUES (N'A', N'C')
INSERT [dbo].[t1] ([SELLER], [NON_SELLER]) VALUES (N'A', N'D')
INSERT [dbo].[t1] ([SELLER], [NON_SELLER]) VALUES (N'B', N'A')
INSERT [dbo].[t1] ([SELLER], [NON_SELLER]) VALUES (N'B', N'C')
INSERT [dbo].[t1] ([SELLER], [NON_SELLER]) VALUES (N'B', N'D')
INSERT [dbo].[t1] ([SELLER], [NON_SELLER]) VALUES (N'C', N'A')
INSERT [dbo].[t1] ([SELLER], [NON_SELLER]) VALUES (N'C', N'B')
INSERT [dbo].[t1] ([SELLER], [NON_SELLER]) VALUES (N'C', N'D')
INSERT [dbo].[t1] ([SELLER], [NON_SELLER]) VALUES (N'D', N'A')
INSERT [dbo].[t1] ([SELLER], [NON_SELLER]) VALUES (N'D', N'B')
INSERT [dbo].[t1] ([SELLER], [NON_SELLER]) VALUES (N'D', N'C')

select non_seller,coupon,sum([sum(BAL)])as [sum(BAL)] from
(
select 
*
from 
(
select t1.non_seller,t2.coupon as coupon,sum(BAL) as [sum(BAL)] from t1  join t2 on T1.SELLER=t2.SELLER group by non_seller,coupon
)x
union
select seller,coupon,0 from t2
) y
group by non_seller,coupon