MySQL如何创建多对多和一对一关系

MySQL如何创建多对多和一对一关系

一、创建多对多

1.学生表

create table students ( id int not null primary key auto_increment, name varchar(45) not null )engine=innodb default charset=utf8;登录后复制

2.课程表

create table courses ( id int not null primary key auto_increment, name varchar(45) not null )engine=innodb default charset=utf8;登录后复制

3.中间表

create table stu_cour ( id int not null primary key auto_increment course_id int not null, stu_id int not null, constraint cour foreign key(course_id) references courses(id), constraint stu foreign key(stu_id) references students(id) )engine=innodb default charset=utf8;登录后复制

4.插入数据

insert into students values (0,"小王");insert into students values (0,"小宋");insert into students values (0,"小李");insert into courses values (0,"语文");insert into courses values (0,"数学");insert into courses values (0,"英语");insert into stu_cour values (0,1,1);insert into stu_cour values (0,1,2);insert into stu_cour values (0,1,3);insert into stu_cour values (0,2,1);insert into stu_cour values (0,2,3);insert into stu_cour values (0,3,2);insert into stu_cour values (0,3,3);登录后复制

5.查询学生1选了哪些科目

SELECT courses.id,courses.name FROM courses INNER JOIN stu_cour ON stu_cour.course_id=courses.idINNER JOIN students ON students.id= 1 and students.id = stu_cour.stu_id;登录后复制

6.查询id=2数学被谁选了

SELECT students.name FROM students INNER JOIN stu_cour ON stu_cour.stu_id =students.idINNER JOIN courses ON courses.id= 2 and stu_cour.course_id = courses.id;登录后复制

二、MySQL 创建一对一关系

1.一对一

创建用户表：

CREATE TABLE users (id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,name VARCHAR(50) NOT NULL)ENGINE=InnoDB DEFAULT CHARSET=utf8;登录后复制

用户信息表：

CREATE TABLE users_info (id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,age int NOT NULL,phone varchar(11) NOT NULL,user_id int not null,constraint user_info foreign key(user_id) references users(id))ENGINE=InnoDB DEFAULT CHARSET=utf8;登录后复制

2.插入数据

insert into users values (0,"小王");insert into users values (0,"小宋");insert into users_info values (0,12,'13812345678',1);insert into users_info values (0,14,'13812345679',2);登录后复制

查询人的全部信息：

select * from users inner join users_info onusers_info.user_id =users.id;登录后复制

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