Database and multi-table foreign key relationships

Field Operations

create table tf1(
    id int primary key auto_increment,
    x int,
    y int
);

# 修改
alter table tf1 modify x char(4) default '';
alter table tf1 change y m char(4) default '';

# 增加
mysql>: alter table 表名 add 字段名 类型[(长度) 约束];  # 末尾
eg>: alter table tf1 add z int unsigned;

mysql>: alter table 表名 add 字段名 类型[(宽度) 约束] first;  # 首位
eg>: alter table tf1 add a int unsigned first;

mysql>: alter table 表名 add 字段名 类型[(宽度) 约束] after 旧字段名;  # 某字段后
eg>: alter table tf1 add xx int unsigned after x;

mysql>: alter table 表名 drop 字段名;  # 删除字段
eg>: alter table tf1 drop a;

Multi-table relations

"""
一对一：丈夫-妻子，用户-身份证，作者-作者详情
一对多：部门-员工，班级-学生，书-出版社
多对多：老师-班级，课程-学生，出版社-作者
"""

# 书 - 出版社 - 作者 - 作者详情 外键分布
# 外键是 建立表与表关联 的字段，通常 一个表的外键 是 另一个表的主键（唯一键也可以）

# 一对一：外键在任何一方都可以，此时外键要设置 唯一键
"""
作者(author)：id，name，sex，age，mobile
作者详情(author_detail)： id，info，address，author_id
----------------------------------------------------
作者(author)：id，name，sex，age，mobile, detail_id
1 Tom 1
2 Bom 2
3 Bob 3

作者详情(author_detail)： id，info，address
1 Tom_info
2 Bom_info
"""

# 一对多：外键必须放在多的一方，此时外键值不唯一
"""
书(book)：id，name，price，publish_id
1 西游记 1
2 东游记 2
3 西厢记 1
4 流浪记 1

出版社(publish)： id，name，address，phone
1 老奶奶出版社
2 小奶奶出版社
"""

# 多对多：一定要创建第三张表(关系表)，每一个外键值不唯一，看可以多个外键建立联合唯一
"""
作者(author)：id, name, age
出版社(publish)：id, name, address
作者与出版社关系表：id, author_id, publish_id
id      author_id       publish_id
1           1               1
2           1               2
3           2               1
4           2               2
"""

Foreign key

# 作者(author)：id，name，sex，age，mobile, detail_id
# 作者详情(author_detail)： id，info，address

# 1、外键的 字段名 可以自定义(名字随意)，通常命名规范(关联表_关联字段)

# 2、外键要通过 foreign key 语法建立表与表之间的关联

# 3、foreign key(所在表的外键字段) references 关联表(关联字段)
# eg：foreign key(detail_id) references author_detail(id)

# 4、级联关系
#   级联更新 on update cascade
#   级联删除 on delete cascade

# 重点：外键字段本身可以唯一或不唯一，但是外键关联的字段一定唯一

One: No cascade relationship

# 作者详情(author_detail)： id，info，address
create table author_detail(
    id int primary key auto_increment,
    info varchar(256),
    address varchar(256)
);

# 作者表id，name，sex，age，mobile, detail_id
create table author(
    id int primary key auto_increment,
    name varchar(64) not null,
    mobile char(11) unique not null,
    sex enum('男', '女') default '男',
    age int default 0,
    detail_id int unique not null,
    foreign key(detail_id) references author_detail(id)
);

# 必须先创建被关联表数据，有关联表外键关联的记录后，关联表才可以创建数据
mysql>: insert into author_detail(info,address) values('Tom_info','Tom_address');
mysql>: insert into author(name,mobile,detail_id) values('Tom','13344556677', 1);
mysql>: insert into author_detail(info,address) values('Bob_info','Bob_address');
mysql>: insert into author(name,mobile,detail_id) values('Bob','15666882233', 2);

# 修改关联表 author
mysql>: insert into author_detail(info,address) values('Tom_info_sup','Tom_address_sup');
mysql>: update author set detail_id=3 where detail_id=2; # 有未被其他数据关联的数据，就可以修改
# 删除关联表 author
mysql>: delete from author where detail_id=3;  # 直接删除

# 修改被关联表 author_detail
mysql>: update author_detail set id=10 where id=1;  # 无法修改
# 删除被关联表 author_detail
mysql>: delete from author_detail where id=1;  # 无法删除

# 没有级联关系下：
# 增加：先增加被关联表记录，再增加关联表记录
# 删除：先删除关联表记录，再删除被关联表记录
# 更新：关联与被关联表都无法完成 关联的外键和主键 数据更新 - (如果被关联表记录没有被绑定，可以修改)

One: A cascade relationship

mysql>: drop table author;
mysql>: drop table author_detail;


# 作者详情(author_detail)： id，info，address
create table author_detail(
    id int primary key auto_increment,
    info varchar(256),
    address varchar(256)
);

# 作者表id，name，sex，age，mobile, detail_id
create table author(
    id int primary key auto_increment,
    name varchar(64) not null,
    mobile char(11) unique not null,
    sex enum('男', '女') default '男',
    age int default 0,
    detail_id int unique not null,
    foreign key(detail_id) references author_detail(id)
    on update cascade 
    on delete cascade
);



# 必须先创建被关联表数据，有关联表外键关联的记录后，关联表才可以创建数据
mysql>: insert into author(name,mobile,detail_id) values('Tom','13344556677', 1);  # 错误
mysql>: insert into author_detail(info,address) values('Tom_info','Tom_address');
mysql>: insert into author(name,mobile,detail_id) values('Tom','13344556677', 1);
mysql>: insert into author_detail(info,address) values('Bob_info','Bob_address');
mysql>: insert into author(name,mobile,detail_id) values('Bob','15666882233', 2);

# 修改关联表 author
mysql>: update author set detail_id=3 where detail_id=2;  # 失败，3详情不存在
mysql>: update author set detail_id=1 where detail_id=2;  # 失败，1详情已被关联
mysql>: insert into author_detail(info,address) values('Tom_info_sup','Tom_address_sup');
mysql>: update author set detail_id=3 where detail_id=2; # 有未被其他数据关联的数据，就可以修改
# 删除关联表 author
mysql>: delete from author where detail_id=3;  # 直接删除

# 修改被关联表 author_detail
mysql>: update author_detail set id=10 where id=1;  # 级联修改，同步关系关联表外键

# 删除被关联表 author_detail
mysql>: delete from author where detail_id=10;  # 可以删除对被关联表无影响
mysql>: insert into author(name,mobile,detail_id) values('Tom','13344556677', 10);
mysql>: delete from author_detail where id=10;  # 可以删除，将关联表的记录级联删除掉

Many

# 一对多：外键必须放在多的一方，此时外键值不唯一

# 出版社(publish)： id，name，address，phone
create table publish(
    id int primary key auto_increment,
    name varchar(64),
    address varchar(256),
    phone char(20)
);

# 书(book)：id，name，price，publish_id, author_id
create table book(
    id int primary key auto_increment,
    name varchar(64) not null,
    price decimal(5, 2) default 0,
    publish_id int,  # 一对多的外键不能设置唯一
    foreign key(publish_id) references publish(id)
    on update cascade
    on delete cascade
);

# 增：先增加被关联表(publish)的数据，再增加关联表(book)的数据
mysql>: insert into publish(name, address, phone) values
('人民出版社', '北京', '010-110'),
('西交大出版社', '西安', '010-119'),
('老男孩出版社', '上海', '010-120');

mysql>: insert into book(name, price, publish_id) values
('西游记', 6.66, 1),
('东游记', 8.66, 1),
('python从入门到入土', 2.66, 2),
('轮程序员修养之道', 3.66, 3),
('好好活着', 88.88, 3);
# 没有被关联的字段，插入依旧错误
mysql>: insert into book(name, price, publish_id) values ('打脸之道', 0.3, 4);  # 失败


# 更新：直接更新被关联表的(publish) 主键，关联表(book) 外键 会级联更新
mysql>: update publish set id=10 where id=1;
# 更新：直接更新关联表的(book) 外键，修改的值对应被关联表(publish) 主键 如果存在，可以更新成功，反之失败
mysql>: update book set publish_id=2 where id=4;  # 成功
mysql>: update book set publish_id=1 where id=4;  # 失败


# 删：
#   删被关联表，关联表会被级联删除
mysql>: delete from publish where id = 2;

#   删关联表，被关联表不会发生变化
mysql>: delete from book where publish_id = 3;

# 假设：书与作者也是 一对多 关系，一个作者可以出版多本书
create table book(
    id int primary key auto_increment,
    name varchar(64) not null,
    price decimal(5, 2) default 0,
    publish_id int,  # 一对多的外键不能设置唯一
    foreign key(publish_id) references publish(id)
    on update cascade
    on delete cascade
    
    # 建立与作者 一对多 的外键关联
    author_id int,  
    foreign key(author_id) references author(id)
    on update cascade
    on delete cascade
);

Many to many

```mysql

Many to many: Be sure to create a third table (table), each foreign key value is not unique, you can see multiple foreign keys to establish a joint unique

mysql>: drop table author;
mysql>: drop table author_detail;
mysql>: drop table book;
mysql>: drop table publish;

Author (author): id, name, age

create table author(
id int primary key auto_increment,
name varchar(64),
age int unsigned default 0
);

Publisher (publish): id, name, address

create table publish(
id int primary key auto_increment,
name varchar(64),
address varchar(256)
);

Authors and publishers table: id, author_id, publish_id

the Table author_publish the Create (
the above mentioned id int Primary Key AUTO_INCREMENT,
# table must have multiple foreign keys, associated with more than one table
# association of table
author_id int,
Foreign Key (author_id) the References author (the above mentioned id)
ON Update Cascade
ON the Delete Cascade,
# associated press table
publish_id int,
Foreign Key (publish_id) the References publish (the above mentioned id)
ON Update Cascade
ON the Delete Cascade,
# establish joint the only two fields
uNIQUE (author_id, publish_id)
);

Note: The table associated with the two tables authors and publishers in the table structure table two key authors and publishers do not have any relationship

By: Two tables are related, there is no context, but the relationship to the relationship table must match after the two tables provide data

MySQL>: INSERT INTO author (name, Age) values ( 'ruakei', 67), ( 'Engo', 76), ( 'of Lxx',. 3);
MySQL>: INSERT INTO publish (name, address) values ( ' Press old boy ',' Shanghai '), (' the little girl publishers', 'Beijing');

Operating table:

mysql>: insert into author_publish(author_id, publish_id) values(1,1),(1,2),(2,1),(2,2),(3,1);

Table operation: add, delete, change, as long as the two are provided with a relation table corresponding to the operation data, can operate successfully and is no effect on the relationship between two tables

Operation is two relational tables:

By: not affect the relationship table

mysql>: insert into publish (name, address) values ​​( 'West Jiaotong University Press', 'Xi');

Change: relational tables are cascading updates

mysql>: update publish set id=10 where id=1;

Delete: Delete table will cascade

mysql>: delete from author where name='ruakei';