查询数据的有关问题

查询数据的问题
有这么一张表，数据类似如下：
col1 col2 col3
a1 b1 c1
a1 b1 c1
a1 b2 c3
a2 b2 c3
a2 b2 c4
a2 b3 c5
a3 b4 c6


现在有如下要求：
以col1为主，查询一张表，表中只要求col1列中的数据不能重复，请问如何查询


[解决办法]

SQL code

------------------------------ Author  :fredrickhu(小F，向高手学习)-- Date    :2011-08-11 21:16:57-- Verstion:--      Microsoft SQL Server 2008 (RTM) - 10.0.1600.22 (Intel X86) --    Jul  9 2008 14:43:34 --    Copyright (c) 1988-2008 Microsoft Corporation--    Enterprise Evaluation Edition on Windows NT 5.1 <X86> (Build 2600: Service Pack 3)--------------------------------> 测试数据：[tb]if object_id('[tb]') is not null drop table [tb]go create table [tb]([col1] varchar(2),[col2] varchar(2),[col3] varchar(2))insert [tb]select 'a1','b1','c1' union allselect 'a1','b1','c1' union allselect 'a1','b2','c3' union allselect 'a2','b2','c3' union allselect 'a2','b2','c4' union allselect 'a2','b3','c5' union allselect 'a3','b4','c6'--------------开始查询--------------------------select *  from  tb t  where  not exists(select 1 from tb where (col1=t.col1 and col2>t.col2) or (col1=t.col1  and col3>t.col3))----------------结果----------------------------/* col1 col2 col3---- ---- ----a1   b2   c3a2   b3   c5a3   b4   c6(3 行受影响)*/