【笔记】4. 添加primary key

-- 方法1
create table mydb1.emp1(
	eid int primary key,
	name varchar(20),
	deptID int,
	salary double
	);
-- 方法2
create table mydb1.emp2(
	eid int,
	name varchar(20),
	deptID int,
	salary double,
	primary key(eid) 
	);
	
insert into mydb1.emp2 values
(1,'zhangsan',1,2000),
(2,'lisi',2,1000),
(3,'wangwu',3,4000);


-- 联合primary key （多者之中至少有一个不一样，并且都不能为空）
create table mydb1.emp3(
	eid int,
	name varchar(20),
	deptID int,
	salary double,
	primary key(name, deptID) 
	);
insert into mydb1.emp3 values
(1,'zhangsan',1,2000),
(2,'lisi',2,1000),
(3,'wangwu',3,4000);
	
# 事后再添加主键
create table mydb1.emp4(
	eid int,
	name varchar(20),
	deptID int,
	salary double
	);
alter table emp4 add primary key(name, deptID);
alter table emp4 drop primary key; #删除主键
drop table mydb1.emp1;
drop table mydb1.emp2, mydb1.emp3;

-- 自增长主键
create table mydb1.t_user1(
	eid int primary key auto_increment,
	name varchar(20),
	deptID int,
	salary double
	);	
insert into mydb1.t_user1 values
(null,'zhangsan',1,2000),
(NULL,'lisi',2,1000),
(null,'wangwu',3,4000);	
# 定义自增长开始值	
create table mydb1.t_user2(
	eid int primary key auto_increment,
	name varchar(20),
	deptID int,
	salary double
	)auto_increment = 100;	
insert into mydb1.t_user2 values
(null,'zhangsan',1,2000),
(NULL,'lisi',2,1000),
(null,'wangwu',3,4000);		

delete from t_user2	#自增长从删除的最后一个开始
truncate t_user2	#同时删除自增长的开始值
	
drop table mydb1.emp4, mydb1.t_user1, mydb1.t_user2;