（四）给表添加字段、修改字段、删除字段

--添加字段
alter table classes add address varchar(50) not null;
--修改字段
alter table classes alter column address varchar(20);
--删除字段
alter table classes drop column address;


--添加多个字段
alter table classes 
add address varchar(22),
    tel varchar(11),
		idCard varchar(3);
		
--判断该列名是否存在，如果存在就删除
if(exists (select * from sys.columns where object_id=object_id('classes') and name='idCard'))
   alter table classes drop column idCard 
go

if(exists (select * from information_schema.columns where table_name='classes' and column_name='tel'))
   alter table classes drop column tel 
go