Mysql创建外键的方法

首先谈一下外键的作用，一是节省空间，二是约束含外键的列中的值，含外键列的值必须为参考列中的id
创建数据表时，创建外键方法

create table userinfo(
   uid bigint auto_increment primary key,
   name varchar(32),
   department_id int,
   xx_id int,
   constraint fk_user_depar foreign key (department_id) references color(id)
  )engine=innodb default charset=utf8;
  
  create table department(
   id bigint auto_increment primary key,
   title char(15)
  )engine=innodb default charset=utf8;

   constraint fk_user_depar foreign key (department_id) references color(id)

外键也可以是两列，但是前提条件是，被参考的表的两列均是主键

CREATE TABLE t5 (
       nid int(11) NOT NULL AUTO_INCREMENT,
       pid int(11) not NULL,
       num int(11),
       primary key(nid,pid)
     ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
 
create table t6(
      id int auto_increment primary key,
      name char(10),
      id1 int,
      id2 int,
      CONSTRAINT fk_t5_t6 foreign key (id1,id2) REFERENCES t1(nid,pid)
     )engine=innodb default charset=utf8;