Foreign key : to establish a relationship between two tables
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Many
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Many to many
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One to One
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Study the relationship between tables and table
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If all data is stored in a table of the drawbacks:
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1. The structure is not clear ----> fatal
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2. waste of space ----> fatal
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3. Scalability poor ----> disadvantages can not be ignored
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Similar to all py python code stored in a file, a strong coupling together ----> ---- decoupled> Split table
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Wendy split solve the above table.
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This time we need above-mentioned "foreign key", two tables to build a strong relationship.
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Confirm the relationship between the table and the table (one to many, many to many, one)
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Note: At this time have to stand in two positions to think, be considered from two tables to establish relations
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Take the position of the employee table as an example: multiple employees can correspond to a department? The answer is obvious: Can! ! ! ! ! ! ! !
Employees and departments: Many
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Then position the department table from thinking of it: multiple departments can correspond to an employee? Is not it.
Many-sector employees to form table
No nonsense, directly on the code
# Create two tables
1. You must first establish the association table, then association table
# Is associated table:
dep:
create table dep(
id int primary key auto_increment,
dep_name varchar(16),
dep_desc varchar(255)
);
# Association Table:
emp:
create table emp(
id int primary key auto_increment,
name varchar(16),
age int,
gender enum('male', 'female', 'others') default 'male',
dep_id int not null,
foreign key(dep_id) references dep(id)
);
# Insert data:
- 1. The insert must be associated with a data table (DEP), and then inserted in the data relation table (EMP) is.
# dep:
insert into dep(dep_name, dep_desc) values
( 'Nb_ Ministry of Foreign Affairs', 'international ambassador sector'),
( 'Sb_ Teaching Department', 'made programmers department !!!!'),
( 'Technology', 'technology limited sector');
# emp:
insert into emp(name, age, gender, dep_id)
values('tank', 17, 'male', 1),
('jason', 70, 'male', 2),
('sean', 50, 'male', 2),
( 'Egon', 88, 'male', 2),
('owen', 95, 'female', 3);
# Error,
insert into emp(name, age, gender, dep_id) values
( 'Cake', 100, 'others', 999);
# Update data or delete data
- update data
# Error
update emp set dep_id=100 where id=2;
# Error
update dep set id=100 where id=1;
# Delete dep_id field is associated, in order to modify the association id field dep table.
delete from emp where id=1;
update dep set id=100 where id=1;
# Delete: delete the records associated with the table, delete the associated record in the table
- Remove the emp table is recorded dep_id 2
delete from emp where dep_id=2;
- and then delete the record id table dep 2
delete from dep where id=2;
- on update cascade
- on delete cascade
- Create a table
# Is associated table:
dep2:
create table dep2(
id int primary key auto_increment,
dep_name varchar(16),
dep_desc varchar(255)
);
# Association Table:
emp2:
create table emp2(
id int primary key auto_increment,
name varchar(16),
age int,
gender enum('male', 'female', 'others') default 'male',
dep_id int not null,
foreign key(dep_id) references dep2(id)
on update cascade
on delete cascade
);
- insert data
# dep:
insert into dep2(dep_name, dep_desc) values
( 'Nb_ Ministry of Foreign Affairs', 'international ambassador sector'),
( 'Sb_ Teaching Department', 'made programmers department !!!!'),
( 'Technology', 'technology limited sector');
# emp:
insert into emp2(name, age, gender, dep_id)
values('tank', 17, 'male', 1),
('jason', 70, 'male', 2),
('sean', 50, 'male', 2),
( 'Egon', 88, 'male', 2),
('owen', 95, 'female', 3);
# Error,
insert into emp(name, age, gender, dep_id) values
( 'Cake', 100, 'others', 999);
- updating data or deleting data
- update record
update dep2 set id=200 where id=1;
- Delete Record
delete from dep2 where id=200;
- using the third table, the establishment of "many to many foreign key relationships" for the two tables.
- book:
create table book(
id int primary key auto_increment,
title varchar(20),
price int,
book_content varchar(255)
);
- author:
create table author(
id int primary key auto_increment,
name varchar(16),
age int
);
- book2author:
create table book2author(
id int primary key auto_increment,
book_id int,
author_id int,
foreign key(book_id) references book(id)
on update cascade
on delete cascade,
foreign key(author_id) references author(id)
on update cascade
on delete cascade
);
- insert data
- book
insert into book(title, price, book_content) values
( 'Golden bottles mei', 199, 'tells the story of the little hazy time'),
( 'Python from entry to breathe', 2000, 'the bald one night to learn how to'),
( 'Trisomy', 200, 'followed by big brother into the universe fantasy world')
;
- author
insert into author(name, age) values
( 'A', 68),
('jason', 88);
- book2author：
insert into book2author(book_id, author_id) values
(1, 1),
(1, 2),
(2, 2),
(3, 1);
# Error, insert data, book_id, author_id must exist
insert into book2author(book_id, author_id) values (4, 4);
# Update or delete
# Update
- update book set price=6666 where id=1;
- update book set id=4 where id=1;
# Delete
- delete from book where id=4;
- user_info:
id, name, age, gender, hobby, id_card
- user:
id , name, age, detail_id（外键）
- detail:
id, gender, hobby, id_card
user with the detail table to establish a one to one foreign key relationships.
foreign key should be built at a higher frequency side.
- Create a table
# Is associated table
create table customer(
id int primary key auto_increment,
name varchar(16),
media varchar(32)
);
# Association Table
create table student(
id int primary key auto_increment,
addr varchar(255),
phone char(11),
id_card char(18),
# Foreign key must be set to a unique
customer_id int unique,
foreign key(customer_id) references customer(id)
on update cascade
on delete cascade
);
- insert data
insert into customer(name, media) values
('hcy', 'facebook'),
( 'Zsb1', 'la');
('zsb2', 'vk'),
( 'Hb', 'beating');
insert into student(addr, phone, id_card, customer_id) values
( 'Shanghai', '15214546711', '440888888888888888', 1),
( 'Beijing' '18888888888', '440777777777777777', 2);
# Error, one to one, one to one relationship must
insert into student(addr, phone, id_card, customer_id) values ('上海', '15214546711', '440888888888888888', 1);