（五）添加、删除约束

--添加新列、约束
alter table classes 
     add number varchar(20) null constraint no_uk unique;   --添加唯一约束：constraint no_uk unique 
--增加主键:primary key 
alter table classes 
     add constraint pk_id primary key(id);		 
--添加外键：foreign key ...references 		 
alter table classes 
     add constraint fk_cid foreign key(cid) references classes(id)  
go 							
--添加唯一约束
alter table classes 
		 add constraint name_uk unique(name);
--添加CHECK约束
alter table classes with nocheck 		 
     add constraint check_age check(age>1);
alter table classes 
     add constraint ck_age check(age>=15 and age<=50)
--添加默认约束	
alter table classes 
     add constraint sex_def default 1 for sex;
--添加一个包含默认值可以为空的列
alter table classes 
     add createDate smalldatetime null 
		 constraint createDate_def default getDate() with values;
									 
									 
-----多个列、约束一起创建-----	
alter table classes add 
     /*添加id主键、自增*/								 
		 id int identity constraint id primary key,
		 /*添加外键约束*/
		 number int null 
		 constraint uNumber references classes(number),
		 /*默认约束*/
		 createDate decimal(3,3)
		 constraint createDate default 2025-02-14
go 

--删除约束
alter table classes drop constraint no_uk;