1、推荐方法
ALTER TABLE t ADD id_column INT IDENTITY (1,1);
eg:
ALTER TABLE [dbo].[Table_test] ADD unique_id INT IDENTITY (1,1) primary key
2.要先删除，后修改
alter table UserInfo drop column ID
alter table UserInfo Add ....
create table UserInfo (
ID int identity(1,1) primary key,
UserName varchar(20) unique not null,
UserPass varchar(20) ,
RegTime datetime,
Email varchar(30)