MySQL创建外键方式

 #创建主表

#年级表（id/年级名称）--主表
CREATE table if not exists grade(
        gradeid int(10) primary key auto_increment,
        gradeName  varchar(50) not NULL
);

#创建外键方式一：创建子表的同时创建外键

#学生信息表（学号、姓名、性别、年级、手机号、地址、出生日期、邮箱、身份证号）--子表
create table if not exists student(
    studentNo int(4) primary key,
    studentName varchar(20) not null default '匿名',
    studentSex tinyint(1) default 1,
    gradeid int(10),
    phone varchar(50) not null,
    address varchar(255),
    borndate datetime,
    email varchar(50),
    identityCard varchar(18) not null,
    constraint fk_gradeid foreign key(gradeid) references grade(gradeid)
);

#创建外键方式二：创建子表完毕后，修改子表添加外键

#首先创建子表

create table if not exists student(
    studentNo int(4) primary key,
    studentName varchar(20) not null default '匿名',
    studentSex tinyint(1) default 1,
    gradeid int(10),
    phone varchar(50) not null,
    address varchar(255),
    borndate datetime,
    email varchar(50),
    identityCard varchar(18) not null
);

#添加外键

alter table student
add constraint fk_gradeid foreign key(gradeid) references grade(gradeid);