sqlserver 修改表欄位預設值，或原欄位不可為空，修改為可以為空白

declare @name varchar(100)
--DF為約束名稱首碼
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%'

--刪除約束
exec('alter table biGuestQuestion drop constraint '+ @name)
ALTER TABLE biGuestQuestion ALTER COLUMN FGuestName varchar(60) null
--為欄位添加新預設值和約束
exec('ALTER TABLE biGuestQuestion ADD CONSTRAINT '+@name +' DEFAULT (-2) FOR [FGuestName]')