*定义主键约束　primary key:  不允许为空，不允许重复
*定义主键自动增长　auto_increment
*定义唯一约束　unique
*定义非空约束　not null
*定义外键约束　constraint ordersid_FK foreign key(ordersid) references orders(id)
*删除主键：alter table tablename drop primary key ;

create table myclass(
  id INT(11) primary key auto_increment,
  name varchar(20) unique
);
create table student (
  id INT(11) primary key auto_increment, 
  name varchar(20) unique,
  passwd varchar(15) not null,
  classid INT(11),                
  constraint stu_classid_FK foreign key(classid) references myclass(id)
  );