[note] May 23, 2022 MySQL

1. Relational database

1.1 Data persistence

Save data to a storage medium that can store data for a long time , In case of power failure, data will not be lost .

1.2 The history of database development

Mesh database 、 Hierarchical database 、 relational database 、NoSQL database 、NewSQL database .

1.3 Relational database features

（1） Theoretical basis ： Relational algebra （ Relationship between operation 、 Set theory 、 First order predicate logic ）.

（2） Concrete representation ： Use two-dimensional table （ There are rows and columns ） Organization data .

（3） programing language ： Structured query language （SQL）.

a. DDL： Data definition language

b. DML： Data operation language

c. DQL： Data query language

d. DCL： Data control language

1.4 ER Model （ Entity relationship model ） And conceptual model diagram

ER Model , Its full name is Entity relationship model （Entity-Relationship Model）, Proposed by Chinese American computer scientist Mr. Chen Pinshan , It is a high-level scanning method of conceptual data model , As shown in the figure below .

1.5 Relational database products

（1）Oracle - At present, the most widely used database management system in the world , As a general database system , It has complete data management function ; As a relational database , It is a product of complete relationship ; As a distributed database , It realizes the function of distributed processing . stay Oracle Abreast of the times 12c In the version , It also introduces a multi tenant Architecture , Using this architecture, you can easily deploy and manage the database cloud .

（2）DB2 - IBM company-developed 、 Mainly operated in Unix（ Include IBM Homemade AIX）、Linux、 as well as Windows Relational database products of server version and other systems .DB2 It has a long history and is considered to be the first to use SQL Database products , It has more powerful business intelligence functions .

（3）SQL Server - from Microsoft Develop and promote relational database products , It was originally applied to the data management of small and medium-sized enterprises , But in recent years, its application scope has expanded , Some large enterprises and even multinational corporations have begun to build their own data management systems based on it .

（4）MySQL - MySQL It's open source , Anyone can be in GPL（General Public License） Download and modify it according to the needs of personalization .MySQL Because of its speed 、 Reliability and adaptability .

（5）PostgreSQL - stay BSD Open source relational database products issued under license .

2. install MySQL

（1） from Official website Download installation package

（2） Follow the installation wizard to install

（3） Verify that the installation was successful and if the installation of section 5 What if the step report is wrong ：

3. Use MySQL

（1） Show all character sets ： show character set;

（2） open MySQL And enter the password , Show Welcome The successful [ Failed to transfer the external chain picture , The origin station may have anti-theft chain mechanism , It is suggested to save the pictures and upload them directly (img-453Tr6Ru-1653297164446)(file:///C:\Users\HP\Documents\Tencent Files\583882690\Image\Group2{%\I9{%I9L52L6U[G35_I6AWP6Y7.jpg)]

（3） Display database ：show databases;

（4） Switch database ：use Database name ;

（5） Display the tables under the database ：show tables;

3.1 DDL

Full name Data Definition Language. Carry out construction 、 Delete 、 Change operation . key word ：create/drop/alter.

（1） Create a database and specify the default character set ：create database Database name default character set utf8mb4;

explain 1：utf8 Said the use of utf-8 Encoding mode ;mb4 Indicates maximum support for 4 Byte character .

explain 2：MySQL8 The default character set is utf8mb4, but MySQL5 The default character set is latin1, So add utf8mb4 To ensure downward compatibility .

（2） Delete database ：drop database if exists school;

（3） Create a 2D table ：

RIGHT Example：

create table tb_student 
(
stu_id int comment ' Student number ', 
stu_name varchar(4) not null comment ' full name ', 
stu_sex boolean default 1 comment ' Gender ', 
stu_birth date comment ' Date of birth ',
primary key (stu_id)
) engine=innodb comment ' Student list ';

explain ：varchar() The parentheses indicate the maximum length of the string ;stu_sex It can also be set to char(1);primary key Indicates adding a primary key constraint , Need unique keys ;engine=innodb Is setting up the engine , It's for downward compatibility

appendix ： Common data types ：

Integers ：integer/int（4 byte ）/tinyint（1 byte ）/smallint（2 byte ）/bigint（8 byte ）

​ int Value range of ：-2^31~231-1

​ int unsigned Value range of ：0~2^32-1（ Don't add... When subtracting unsigned）

decimal ：decimal(10,2) Expressing common ownership 10 Significant digits , There are at most after the decimal point 2 position

date ：date/time/datetime. notes ：timestamp（ Time stamp ,2038 The annual meeting overflowed , It is not recommended to use ）

Text ：varchar. notes ：char comparison varchar No advantage , Not recommended for use

（4） View table structure ：desc tb_student;

（5） Delete 2D table ：drop table if exists tb_student;

（6） Modify the 2D table ：

a. Add a column

alter table tb_student add column stu_addr varchar(100) default '';
alter table tb_student add column tel varchar(20) not null;

b. Delete a column

alter table tb_student drop column tel;

c. Modify a column

alter table tb_student change column stu_sex stu_gender boolean default 1;
alter table tb_student modify column stu_sex char(1) default 'M';

notes ： Change your name to change, A new data type must be added later ; Only change the data type to use modify.

d. Adding constraints

alter table tb_student add constraint primary key (stu_id);

e. Change the name of the table （ Do not easily change the names of libraries and tables ）

alter table tb_student rename to ...;

appendix ： Copy a table ：