这句 sql怎么写？

Question

--建立测试环境
Create Table 表(CA varchar(10),CB varchar(10))
--插入数据
insert into 表
select '1','a' union
select '1','b' union
select '2','a' union
select '2','b' union
select '2','c' union
select '3','d' union
select '3','e'
select * from 表
--测试语句
go
CREATE FUNCTION FunMergeCharField(@vchA varchar(10))
RETURNS varchar(8000)
AS
BEGIN
DECLARE @r varchar(8000)
SET @r=''
SELECT @r=@r+','+CB FROM 表 WHERE CA=@vchA 
RETURN(substring(@r,2,8000))
END
GO 
select ca,dbo.FunMergeCharField(ca)as cb from 表 group by ca
--删除测试环境
Drop Table 表
drop FUNCTION FunMergeCharField 
/*  
ca         cb                                                                                                                                                                                                                                                              
---------- ----------
1          a,b
2          a,b,c
3          d,e
*/