foreign key: 外键约束---如果一张表中的一个字段指向了另一张表中的主键，就将该字段叫做外键；外键的数据类型必须和指向的主键一致；定义外键的表称为从表，被外键引用的表称为主表

create table if not exists user(

    id int unsigned primary key auto_increment,

    username varchar(20) not null

)

desc user;

create table if not exists dynamic(

    id int unsigned primary key auto_increment,

    content varchar(255) not null,

    user_id int unsigned

)

create table if not exists dynamic_1(

    id int unsigned primary key auto_increment,

    content varchar(255) not null,

    user_id int unsigned,

    foreign key(user_id) references user(id)

)

create table if not exists dynamic_2(

    id int unsigned primary key auto_increment,

    content varchar(255) not null,

    user_id int unsigned,

    foreign key(user_id) references user(id) on update cascade on delete set null

)