MySQL ---- multi-table operations

The relationship between the ## multi-table

1-one (understand)

　　* Such as: people and identity cards

　　* Analysis: A person is only an ID card, an ID card corresponds to only one person.

2, many (many)

　　* Such as: departments and employees

　　* Analysis: The department has a staff of more than one employee corresponds to only one department.

3-many

　　* Such as: students and courses

　　* Analysis: A lot of students can choose their courses, a course can also be the choice of many students.

Realization relationship

1-to-many (many)

　　* Such as: departments and employees

　　* Implementation: establishing a foreign key in a multi-party, one of the points to a primary key.

 

 

 2-many

　　* Such as: departments and employees

　　* Ways: by means of the third intermediate table comprising at least two fields, the two fields third table as a foreign key of the primary key of each of the two primary table.

 

 

3, one on one

　　*  Such as: people and identity cards

　　* Implementation: You can add foreign key refers unique primary key in any other one.

Case:

/ * Tourism Case
  1, classification table and the table is a line-to-many relationship
  2, the line is many to many tables and user tables, creating an intermediate table.
  */
create table category(
    cid int primary key auto_increment,
    name varchar(64) not null unique
);
create table routes(
    rid int primary key auto_increment,
    name varchar(128) not null unique ,
    price double,
    cid int,
    /*constraint rou_cat_fk */foreign key (cid) references category(cid)
                   on delete cascade on update cascade
);
create table user(
    uid int primary key auto_increment,
    username varchar(32) not null unique ,
    password varchar(32) not null
);
create table favorite(
    rid int,
    date datetime,
    uid int,
    /*创建复合主键*/
    primary key (rid,uid),
    foreign key (rid) references routes(rid),
    foreign key (uid) references user(uid)
);
insert into category(cid, name) VALUES (1,'国外游'),(2,'国内游');
insert into routes(rid, name, price, cid) VALUES (1,'希腊游',10000.1,1),(2,'北京游',9992.1,2);
insert into user (uid, username, password)
values (null,'ftj','123'),(null,'lxy','456');
insert into favorite (rid, date, uid)
values (1,'2020-02-08 23:59:59',1),(2,'2020-02-08 23:59:59',2);
select * from user where uid = (select uid from favorite where rid = 1);