如何改變一個字段的缺省值？

--缺省對象名稱
declare @df_name sysname;
--缺省值
declare @df_value varchar(50);

select 
    @df_name=object_name(sm.id),
    @df_value=sm.text
from dbo.sysobjects so 
    JOIN dbo.syscolumns sc 
on SO.id = SC.id
    JOIN dbo.syscomments sm 
on SC.cdefault = sm.id
where SO.xtype = 'u'
    AND SO.name='tablename'
    AND SC.name='colname';
select @df_name,@df_value

--先去掉該缺省值
exec('alter table tablename drop constraint ' + @df_name);
--再添加新的缺省值
exec('alter table tablename add constraint ' + @df_name + '

 default 0 for colname');