请问一个数据有嵌套的查询语句

请教一个数据有嵌套的查询语句
有表数据如下：
a b
--------------------
11 1
11 2
12 11
12 3
13 11
13 12

执行查询a=13的结果：
a b
--------------------
13 1
13 2
13 3

[解决办法]

SQL code

----------------------------------------  Author : htl258(Tony)--  Date   : 2010-04-27 11:25:06--  Version:Microsoft SQL Server 2008 (RTM) - 10.0.1600.22 (Intel X86) --          Jul  9 2008 14:43:34 --          Copyright (c) 1988-2008 Microsoft Corporation--          Developer Edition on Windows NT 5.1 <X86> (Build 2600: Service Pack 3)--  Blog   : http://blog.csdn.net/htl258----------------------------------------> 生成测试数据表:tbIF NOT OBJECT_ID('[tb]') IS NULL    DROP TABLE [tb]GOCREATE TABLE [tb]([a] INT,[b] INT)INSERT [tb]SELECT 11,1 UNION ALLSELECT 11,2 UNION ALLSELECT 12,11 UNION ALLSELECT 12,3 UNION ALLSELECT 13,11 UNION ALLSELECT 13,12GO--SELECT * FROM [tb]-->SQL查询如下:declare @i intset @i=13;with t as(    select * from tb where a=@i    union all    select a.* from tb a join t b on b.b=a.a)select distinct @i a,b from t a where not exists(select 1 from t where a.b=a)order by b/*a           b----------- -----------13          113          213          3(3 行受影响)*/