MySQL实战：主键，primary key，唯一，非空
1.创建表
create table test_primary_key(
id int unsigned primary key,
username varchar(20)
);
复合主键
create table test_primary_key3(
id int unsigned,
courseid int unsigned,
username varchar(20),
email varchar(50),
primary key(id,courseid)
);
2.插入数据
(1)insert test_primary_key(id,username) values(1,'king');
(2)insert test_primary_key(id,username) values(1,'king123');
(3)insert test_primary_key(username) values('Queue');
【主键primary key特性】：非空，唯一
