sql 多行同一字段 拼接字符串

我通过SELECT查出了符合条件的列，现在我要把它拼接成一个字符串怎么做
比如字段 A
        DDD
        AAA
我现在要拼接成一条字符串中间不能有，号

---

SQL code:
declare @s varchar(100)
select @s=isnull(@s+'','')+col from tb
select @s

---

SQL code:
--> 生成测试数据表:tb

If not object_id('[tb]') is null
Drop table [tb]
Go
Create table [tb]([A] nvarchar(3))
Insert [tb]
Select N'DDD' union all
Select N'AAA'
Go
--Select * from [tb]

-->SQL查询如下:
declare @s varchar(50)
select @s=isnull(@s,'')+A from tb
select @s
/*
--------------------------------------------------
DDDAAA

(1 行受影响)
*/

---

SQL code:
-->Title:Generating test data
-->Author:wufeng4552
-->Date :2009-09-30 15:41:31

if not object_id('tb') is null
drop table tb
Go
Create table tb([test] nvarchar(3))
Insert tb
select N'A' union all
select N'DDD' union all
select N'AAA'
Go
declare @s varchar(100)
select @s=isnull(@s+'','')+test from tb
select @s
/*
-------------------------------------------------------------
ADDDAAA

*/

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