如何直接用sql删除constraint
在查询分析器里用如下语句：
declare   @mylen   int,
@dropname   char(100)
set   @mylen=(SELECT   length   FROM   syscolumns   WHERE   name   =   'fwxzdh '   AND   id   =   (SELECT   id   FROM   sysobjects   WHERE   name   =   'tb_fwhd_wxz '))

set   @dropname=(select   rtrim(name)   as   dropname   from   sysobjects   where   xtype= 'D '   and   parent_obj=(select   id   from   sysobjects   where   name= 'tb_fwhd_wxz '   and   xtype= 'U ')   and   id   in   (select   cdefault   from   syscolumns   where   name= 'fwxzdh '))

if   len(@dropname)> 0   and   @mylen <> 8  
alter   table   tb_fwhd_wxz   drop   constraint   @dropname
alter   table   tb_fwhd_wxz   alter   column   fwxzdh   char(8)   not   null

ALTER   TABLE   [dbo].[tb_fwhd_wxz]   WITH   NOCHECK   ADD   CONSTRAINT   [DF_tb_fwhd_wxz_fwxzdh_20070920]   default(‘’)   for   [fwxzdh]

主要是判断fwxzdh字段长度是否为8，如果不是先删除默认值限制，再修改字段长度，最后再加上默认值限制

运行时出下面的错误:
服务器:   消息   170，级别   15，状态   1，行   8
第   8   行:   '@dropname '   附近有语法错误。

因为@dropname是变量，而直接在sql语句中不能识别变量，所以出错，请问各位如何解决

非常感谢！！！

------解决方案--------------------
declare @mylen int,
--@dropname char(100)
@dropname sysname

--set @mylen=(SELECT length FROM syscolumns WHERE name = 'fwxzdh ' AND id = (SELECT id FROM sysobjects WHERE name = 'tb_fwhd_wxz '))
SELECT @mylen=length FROM syscolumns WHERE name = 'fwxzdh ' AND id = object_id( 'tb_fwhd_wxz ')

--这一句自己改
set @dropname=(select rtrim(name) as dropname from sysobjects where xtype= 'D ' and parent_obj=(select id from sysobjects where name= 'tb_fwhd_wxz ' and xtype= 'U ') and id in (select cdefault from syscolumns where name= 'fwxzdh '))

--if len(@dropname)> 0 and @mylen <> 8
if @dropname is not null and @mylen <> 8
--alter table tb_fwhd_wxz drop constraint @dropname
exec ( 'alter table tb_fwhd_wxz drop constraint ' + @dropname)

alter table tb_fwhd_wxz alter column fwxzdh char(8) not null

------解决方案--------------------
chenyuandxm(一剑平江湖) ( ) 信誉：99 2007-09-20 17:14:28 得分: 0


alter table tb_fwhd_wxz drop constraint @dropname
就是这句话有问题，删除时不能用变量
============================================================================
declare @sql nvarchar(2000)
select @sql = N 'alter table tb_fwhd_wxz drop constraint ' + @dropname
exec sp_executesql @sql