请教个sql语句

我想查询下一个表中的某字段是否有重复值，sql该怎么写呢？
谢谢

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SQL code:
----------------------------------------------------------------
-- Author :fredrickhu(小F，向高手学习)
-- Date :2009-11-07 14:30:22
-- Version:
-- Microsoft SQL Server 2005 - 9.00.4035.00 (Intel X86)
-- Nov 24 2008 13:01:59
-- Copyright (c) 1988-2005 Microsoft Corporation
-- Developer Edition on Windows NT 5.1 (Build 2600: Service Pack 3)
--
----------------------------------------------------------------
--> 测试数据：[tb]
if object_id('[tb]') is not null drop table [tb]
go
create table [tb]([col] int)
insert [tb]
select 1 union all
select 1 union all
select 2 union all
select 3 union all
select 4 union all
select 5 union all
select 6
--------------开始查询--------------------------

select col from [tb] group by col having count(1)>1
----------------结果----------------------------
/* col
-----------
1

(1 行受影响)
*/

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1楼的这样查的话可以说明字段中1是重复值

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SQL code:

select count(x.a) as 列数1from table x

select count( distinct (x.a)) as 列数2 from table x




--------------
列数1与列数2 不相等时 table 中有重复字段。

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SQL code

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Code h