sqlserver 修改表字段默认值，或原字段不能为空，修改为可以为空

declare @name varchar(100)
--DF为约束名称前缀
select @name=b.name from syscolumns a,sysobjects b where a.id=object_id('biGuestQuestion')
and b.id=a.cdefault and a.name='FGuestName' and b.name like 'DF%'

--删除约束
exec('alter table biGuestQuestion drop constraint '+ @name)
ALTER TABLE biGuestQuestion ALTER COLUMN FGuestName varchar(60) null
--为字段添加新默认值和约束
exec('ALTER TABLE biGuestQuestion ADD CONSTRAINT '+@name +' DEFAULT (-2) FOR [FGuestName]')