Installation and use of MySQL under MySQL 19 Linux

One . Instructions before installation

1.1. Linux Preparation of systems and tools

1. Install and start two virtual machines ： CentOS 7.
2. Installed with Xshell and Xftp Wait for a visit CentOS System tools .
3. CentOS6 and CentOS7 stay MySQL Differences in the use of

1.  A firewall ：6 yes iptables,7 yes firewalld
2.  The command to start the service ：6 yes service,7 yes systemctl

1.2. Check to see if MySQL

1. If you use rpm install , Check the RPM PACKAGE：

rpm -qa | grep -i mysql # -i  Ignore case 

2. Check mysql service：

systemctl status mysqld.service

3. If there is mysql-libs Old version of package , It is shown as follows ：
   
4. If it doesn't exist mysql-lib Version of , It is shown as follows ：
   

1.3. MySQL The uninstall

1. close mysql service

systemctl stop mysqld.service;

2. View the current mysql Installation status

rpm -qa | grep -i mysql;
 perhaps 
yum list installed | grep mysql;

3. Uninstall the installed programs found by the above command

yum remove mysql-xxx mysql-xxx mysql-xxx mysqk-xxxx

Be sure to unload clean , Repeat rpm -qa | grep -i mysql Confirm whether there are unloading residues .

4. Delete mysql Related documents
   Find related files

find / -name mysql

Delete the relevant files found by the above command

rm -rf xxx;

5. Delete my.cnf

rm -rf /etc/my.cnf

Two . MySQL Of Linux Version installation

2.1. MySQL Of 4 The big version

1. MySQL Community Server Community version , Free open source , Free download , But no official technical support , Suitable for most ordinary users .
2. MySQL Enterprise Edition Enterprise version , Pay for , Can't download online , You can try 30 God . It provides more functions and more complete technical support , It is more suitable for enterprise customers with high requirements for database function and reliability .
3. MySQL Cluster Cluster version , Free open source . It is used to set up cluster servers , You can put a few MySQL Server Encapsulate into a Server. It needs to be used on the basis of community version or enterprise version .
4. MySQL Cluster CGE Advanced clustering , Pay for .
   Besides , Officials also provide MySQL Workbench （GUITOOL） One for MySQL The design of the ER/ Database modeling tools . It is a famous database design tool DBDesigner4 Successor .MySQLWorkbench It's divided into two versions , Namely Community Edition （MySQL Workbench OSS）、 Commercial version （MySQL WorkbenchSE）.

2.2. download MySQL Specify the version

1. Download address
   Official website ： https://www.mysql.com
2. Open the website , Click on DOWNLOADS
   then , Click on MySQL Community(GPL) Downloads
   
3. Click on MySQL Community Server
   
4. stay General Availability(GA) Releases Select the appropriate version
   If installed Windows Under the system MySQL , Recommended download MSI Erection sequence ; Click on Go to Download Page Download it
   
   Windows Under the MySQL There are two installers installed
   mysql-installer-web-community-8.0.25.0.msi Download program size ：2.4M; Networking installation components are required during installation .
   mysql-installer-community-8.0.25.0.msi Download program size ：435.7M; You can install offline during installation . recommend .
5. Linux Installation under system MySQL In several ways :
   5.1. Linux There are three common ways to install software under the system ：
   The way 1：rpm command
   Use rpm The command installation extension is ".rpm" Software package .
   .rpm The general format of the package ：
   
   The way 2：yum command
   Networking required , from Internet access Of yum Source , Use it directly yum Command to install .
   The way 3： Compile and install source code package
   in the light of tar.gz This compression format , Use tar Command to unzip ; If it's any other compression format , Just use other commands .
   5.2. Linux Installation under system MySQL, The official gives a variety of installation methods

Installation mode	characteristic
rpm	Simple installation , Poor flexibility , Unable to flexibly select the version 、 upgrade
rpm repository	The installation package is very small , Version installation is simple and flexible , Easy upgrade , It needs to be installed online
General binary package	Installation is more complicated , High flexibility , The platform has good universality
Source package	Installation is the most complicated , Long time , The parameter setting is flexible , Good performance

You can't directly choose CentOS 7 The version of the system , So choose the corresponding Red Hat Enterprise Linux
https://downloads.mysql.com/archives/community/ Direct point Download download RPM Bundle Full package . Includes all the following components . You don't need to download one by one .

6. Download the tar package , Open it with a compressor
   
   After decompressing rpm Installation package （ The red box is the extracted installation package ）

2.3. CentOS7 Lower inspection MySQL rely on

1. Check /tmp Temporary directory permissions （ essential ）
   because mysql During installation , Will pass mysql The user is in /tmp New under the directory tmp_db file , So please give /tmp Greater authority . perform ：

chmod -R 777 /tmp

2. Before the installation , Checking depends on

rpm -qa|grep libaio

If there is net-tools The package is as follows ：

rpm -qa|grep net-tools

If there is no need to go to centos In the installation disk rpm install . install linux If there is a graphical interface , These are all installed .

2.4. CentOS7 Next MySQL setup script

Copy the installer to /opt Under the table of contents
stay mysql The installation file directory of ：（ It must be carried out in order ）

rpm -ivh mysql-community-common-8.0.26-1.el7.x86_64.rpm 
rpm -ivh mysql-community-client-plugins-8.0.26-1.el7.x86_64.rpm
rpm -ivh mysql-community-libs-8.0.26-1.el7.x86_64.rpm 
rpm -ivh mysql-community-client-8.0.26-1.el7.x86_64.rpm
rpm -ivh mysql-community-server-8.0.26-1.el7.x86_64.rpm

notes ：

1. When executed rpm -ivh mysql-community-libs-8.0.26-1.el7.x86_64.rpm There is an error ： Dependency detection failed ：mariadb-libs By mysql-community-libs-8.0.26-1.el7.x86_64 When replacing errors , perform ：

yum remove mysql-libs

2. When executed rpm -ivh mysql-community-server-8.0.26-1.el7.x86_64.rpm appear ：

 error ： Dependency detection failed ：
	/usr/bin/perl  By  mysql-community-server-8.0.26-1.el7.x86_64  need 
	net-tools  By  mysql-community-server-8.0.26-1.el7.x86_64  need 
	perl(Getopt::Long)  By  mysql-community-server-8.0.26-1.el7.x86_64  need 
	perl(strict)  By  mysql-community-server-8.0.26-1.el7.x86_64  need 

perform ：

yum install net-tools
yum install -y perl-Module-Install.noarch

Then install ：

rpm -ivh mysql-community-server-8.0.26-1.el7.x86_64.rpm

• . Be careful : For example, when checking the work , No inspection mysql Environment dependent installation mysql-community-server Will report a mistake .
• pm yes Redhat Package Manage abbreviation , adopt RPM Management of , Users can wrap the source code as rpm Is the file form of the extension , Easy to install .
• -i , --install Install package .
• -v , --verbose Provide more detailed information output .
• -h , --hash The hash mark is listed when the package is installed ( and -v The effect is better when used together ), Show the progress bar .

2.5. see MySQL edition

Execute the following command , If successful, it means that the installation is mysql success . similar java -version If you print the version and other information .

mysql --version
# or 
mysqladmin --version

Execute the following command , Check whether the installation is successful . Need to increase the -i Don't be case sensitive , Otherwise we can't find .

rpm -qa|grep -i mysql

2.6. Initialization of services

In order to ensure that the owner of database directories and files is mysql The logged in user , If you are root Identity running mysql service , You need to perform the following command initialization ：

mysqld --initialize --user=mysql

bright ： --initialize Options default to “ Security ” Mode to initialize , Will be root The user generates a password and will The password is marked as expired , After logging in, you need to set a new password . Generated Temporary password Will record a copy in the log .
Check the password ：

cat /var/log/mysqld.log

[email protected]: Followed by the initialization password

2.7. start-up MySQL, Check the status

# Add do not add .service Suffixes are OK 
 start-up ：systemctl start mysqld.service
 close ：systemctl stop mysqld.service
 restart ：systemctl restart mysqld.service
 Check the status ：systemctl status mysqld.service

mysqld  This executable represents  MySQL  Server program , Running this executable file can directly start a 
 Server process .

Check the process ：

2.8. see MySQL Whether the service starts automatically

systemctl list-unit-files | grep mysqld.service

The default is enabled.

• If not enabled You can run the following command to set the auto start

systemctl enable mysqld.service

• If you want to avoid self starting , Run the following command

systemctl disable mysqld.service

3、 ... and . MySQL Sign in

3.1. For the first time login

adopt mysql -hlocalhost -P3306 -uroot -p Log in , stay Enter password： Enter the initialization password

3.2. Change Password

1. Because the initialization password expires by default , So checking the database will report an error .
2. Change Password ：

alter user 'root'@'localhost' identified by '123456';

3. 5.7 After the version （ Not included 5.7）,mysql Added a new password security mechanism . Setting a new password is too simple and will report an error .

3.3. Set up remote login

3.3.1. The current problem

In use SQLyog or Navicat Configure remote connections in Mysql The following error messages are encountered when the database , This is because Mysql Configured... Caused by unsupported remote connection .

3.3.2. Confirm the network

1. Use... On remote machines ping ip Address Make sure the network is smooth .

2. Use... On remote machines telnet command Ensure that the port number is open visit .
   

3. expand ： telnet Command on
   

3.3.3. Turn off the firewall or open ports

Mode one ： Turn off firewall
CentOS6 ：

service iptables stop

CentOS7：

systemctl start firewalld.service

systemctl status firewalld.service

systemctl stop firewalld.service

# Set boot to enable firewall 
systemctl enable firewalld.service

# Set boot disable firewall 
systemctl disable firewalld.service

Mode two ： Open ports

• Check the open port number

firewall-cmd --list-all

• Set the open port number

firewall-cmd --add-service=http --permanent
firewall-cmd --add-port=3306/tcp --permanent

• service iptables restart

firewall-cmd --reload

3.4. Linux Modify the configuration under

stay Linux System MySQL Next test ：

use mysql;
select Host,User from user;

You can see root The current host configuration information of the user is localhost.
modify Host For wildcard %
ost Column specifies the IP, such as user=root Host=192.168.1.1. This means that root Users can only pass through 192.168.1.1 Client access . user=root Host=localhost, It means that it can only be accessed through the local client . and % It's a wildcard , If Host=192.168.1.%, So that means as long as it's IP Address prefix is “192.168.1.” All clients of can connect . If Host=% , Express all IP All have connection permission .
Be careful ： In a production environment, you can't save host Set to %, There will be security problems , Specific settings can be set according to the production environment IP Set it up .

update user set host = '%' where user ='root';

Host Set up “%” Then you can allow remote access .
Host Remember to execute after modification flush privileges Make the configuration take effect immediately ：

flush privileges;

3.5. test

• If it is MySQL5.7 edition , You can use SQLyog perhaps Navicat Successfully connected to MySQL 了 .
• If it is MySQL8 edition , The following problems also occur when connecting ：
  Configuration new connection error ： Wrong number 2058, The analysis is mysql Password encryption has changed .
  resolvent ：Linux Next mysql -u root -p Log in to your mysql database , then Implement this SQL：

ALTER USER 'root'@'%' IDENTIFIED WITH mysql_native_password BY '123456';

And then reconfigure SQLyog The connection of , The connection is successful ,OK.

Four . MySQL8 Password strength evaluation （ understand ）

4.1. MySQL Set passwords for different versions ( May appear )

1. MySQL5.7 in ： success

mysql> alter user 'root' identified by 'abcd1234';
Query OK, 0 rows affected (0.00 sec)

2. MySQL8.0 in ： Failure

mysql> alter user 'root' identified by 'abcd1234'; 
ERROR 1819 (HY000): Your password does not satisfy the current policy requirements

4.2. MySQL8 Previous security policy

stay MySQL 8.0 Before ,MySQL It uses validate_password Plug in detection 、 Verify account and password strength , Guarantee the security of the account .

1. install / Enable plug-in mode 1： In the parameter file my.cnf Add parameters in .

[mysqld]
plugin-load-add=validate_password.so
\#ON/OFF/FORCE/FORCE_PLUS_PERMANENT:  Whether to use the plug-in ( And mandatory / Permanent mandatory use )
validate-password=FORCE_PLUS_PERMANENT

 explain 1： plugin library Medium validate_password The suffix of the file name varies according to the platform .  about Unix and 
Unix-like In terms of system , Its file suffix is .so, about Windows In terms of system , Its file suffix is .dll.
 explain 2：  After modifying the parameters, you must restart MySQL Service to take effect .
 explain 3：  Parameters FORCE_PLUS_PERMANENT To prevent the plug-in from MySQL It is unloaded at runtime . When you uninstall the plug-in, you will report an error . As shown below .

mysql> SELECT PLUGIN_NAME, PLUGIN_LIBRARY, PLUGIN_STATUS, LOAD_OPTION
-> FROM INFORMATION_SCHEMA.PLUGINS
-> WHERE PLUGIN_NAME = 'validate_password';

mysql> UNINSTALL PLUGIN validate_password;
ERROR 1702 (HY000): Plugin 'validate_password' is force_plus_permanent and can not be
unloaded
mysql>

2. install / Enable plug-in mode 2： Run time command installation （ recommend ）

 INSTALL PLUGIN validate_password SONAME 'validate_password.so';
  perhaps 
 INSTALL COMPONENT 'file://component_validate_password';

The metadata will also be registered to this method , That is to say mysql.plugin In the table , So don't worry MySQL After restart, the plug-in will fail .

4.3. MySQL8 Security policy

4.3.1. validate_password explain

MySQL 8.0, The introduction of server components （Components） This feature ,validate_password The plug-in has been re implemented with server components
present .8.0.25 Version of the database , Auto install by default validate_password Components .
Before installing the plug-in , Execute the following two instructions , Execution effect ：

 show variables like 'validate_password%';
 SELECT * FROM mysql.component;

After installing the plug-in , Execute the following two instructions , Execution effect ：

About validate_password Description of system variables corresponding to components ：

Options	The default value is	Parameters to describe
validate_password_check_user_name	ON	Set to ON The password can be set to the current user name .
validate_password_dictionary_file		The pathname of the dictionary file used to check the password , The default is empty.
validate_password_length	8	Minimum password length , In other words, the password length must be greater than or equal to 8
validate_password_mixed_case_count	1	If the password policy is medium or stronger ,validate_password Minimum number of lowercase and uppercase characters required for passwords . For a given value, the password must have so many lowercase characters and so many uppercase characters .
validate_password_number_count	1	The number of digits the password must contain
validate_password_policy	MEDIUM	Password strength level , You can use numerical values 0、1、2 Or the corresponding symbolic value LOW、MEDIUM、STRONG To specify the . 0/LOW ： Check length only .1/MEDIUM ： Check the length 、 Numbers 、 Case write 、 Special characters . 2/STRONG ： Check the length 、 Numbers 、 Case write 、 Special characters 、 Dictionary file .
validate_password_special_char_count	1	The number of special characters a password must contain

Tips ：
The default values of components and plug-ins may be different . for example ,MySQL5.7.validate_password_check_user_name The default value is OFF.

4.3.2. Modify security policy

Modify password verification security strength

SET GLOBAL validate_password_policy=LOW;
SET GLOBAL validate_password_policy=MEDIUM;
SET GLOBAL validate_password_policy=STRONG;
SET GLOBAL validate_password_policy=0; # For LOW
SET GLOBAL validate_password_policy=1; # For MEDIUM
SET GLOBAL validate_password_policy=2; # For HIGH

# Be careful , If it's a plug-in ,SQL by set global validate_password_policy=LOW

Besides , You can also change the length of characters in the password

set global validate_password_length=1;

4.3.3. Password strength test

If you create a password, you encounter “Your password does not satisfy the current policy requirements”, The function component can be used to detect whether the password meets the conditions ： 0-100. When the assessment is in 100 It means that the most basic rules are used ： Capitalization + A lowercase letter + Special characters + It's made up of numbers 8 Password above .

SELECT VALIDATE_PASSWORD_STRENGTH('medium');

SELECT VALIDATE_PASSWORD_STRENGTH('K354*45jKd5');

Be careful ： If not installed validate_password Components or plug-ins , Then this function always returns 0. On the password complexity strategy corresponding to password complexity . As shown in the table below ：

Password Test	Return Value
Length < 4	0
Length ≥ 4 and < validate_password.length	25
Satisfies policy 1 (LOW)	50
Satisfies policy 2 (MEDIUM)	75
Satisfies policy 3 (STRONG)	100

4.4. Uninstall plugins 、 Components ( understand )

Uninstall plugins

UNINSTALL PLUGIN validate_password;

Uninstall components

 UNINSTALL COMPONENT 'file://component_validate_password';

5、 ... and . Related operations of character set

5.1. modify MySQL5.7 Character set

5.1.2. Modification steps

stay MySQL 8.0 Before the release , The default character set is latin1 ,utf8 The character set points to utf8mb3 . Website developers often change the code to utf8 Character set . If you forget to change the default code , There will be a problem of garbled code . from MySQL8.0 Start , The default encoding of the database will be changed to utf8mb4 , So as to avoid the above problem of garbled code .
operation 1： View the default character set

show variables like 'character%';
#  perhaps 
show variables like '%char%'

MySQL8.0 In the implementation of ：
MySQL5.7 In the implementation of ：

MySQL 5.7 The default client and server use latin1 , No Chinese support , Saving Chinese will report an error .MySQL5.7 The screenshot is as follows ：
stay MySQL5.7 When adding Chinese data to , Report errors ：

create database dbtest1;

create table emp1(id int,lname varchar(15));
show tables;

insert into emp1 values (1,' Zhang San ');

Because by default , The table is created using latin1 . as follows ：

show create table emp1;

operation 2： Modify character set

vim /etc/my.cnf

stay MySQL5.7 Or in previous versions , Add Chinese character set configuration at the end of the file ：

character_set_server=utf8

operation 3： Restart MySQL service

systemctl restart mysqld

notes ： But the original library 、 The setting of the original table will not change , Parameter modification is only effective for the newly created database .

5.1.2. Existing library & Table character set changes

MySQL5.7 In the version , Previously created Libraries , The character set of the created table is still latin1.

Modify the character set of the created database

alter database dbtest1 character set 'utf8';

Modify the character set of the created data table

alter table emp1 convert to character set 'utf8';

Add data

insert into emp1 values(2,' Zhang San ');

Be careful ： But if the original data is not used ’utf8’ Code words , The encoding of the data itself will not change . Existing data needs to be exported or deleted , And then re insert .

5.2. Character sets at all levels

MySQL Yes 4 Two levels of character sets and comparison rules , Namely ：

1.  Server level 
2.  Database level 
3.  Table level 
4.  Column level 

The implementation is as follows SQL sentence ：

show variables like 'character%';

1. character_set_server： Server level character set .
2. character_set_database： The character set of the current database .
3. character_set_client： The character set used by the server to decode the request .
4. character_set_connection： When the server processes the request, it will change the request string from character_set_client To character_set_connection.
5. character_set_results： The character set used by the server to return data to the client .

5.2.1. Server level

character_set_server ： Server level character set .
We can use the startup option when starting the server program or when the server program is running SET Statement to modify the values of these two variables . For example, we can write this in the configuration file ：

[server]
character_set_server=gbk #  Default character set 
collation_server=gbk_chinese_ci # The corresponding default comparison rule 

After reading the configuration file when the server starts, the values of these two system variables are modified .

5.2.2. Database level

character_set_database ： The character set of the current database .
When we create and modify a database, we can specify the character set and comparison rules of the database , The specific syntax is as follows ：

CREATE DATABASE  Database name 
[[DEFAULT] CHARACTER SET  Character set name ]
[[DEFAULT] COLLATE  Comparison rule name ];

ALTER DATABASE  Database name 
[[DEFAULT] CHARACTER SET  Character set name ]
[[DEFAULT] COLLATE  Comparison rule name ];

5.2.3. Table level

We can also specify the character set and comparison rules of the table when creating and modifying the table , The grammar is as follows ：

CREATE TABLE  Table name  ( Column information )
[[DEFAULT] CHARACTER SET  Character set name ]
[COLLATE  Comparison rule name ]]
ALTER TABLE  Table name 
[[DEFAULT] CHARACTER SET  Character set name ]
[COLLATE  Comparison rule name ]

If the statement creating and modifying the table does not specify the character set and comparison rules , The character set and comparison rule of the database where the table is located will be used as the character set and comparison rule of the table .

5.2.4. Column level

For columns that store strings , Different columns in the same table can have different character sets and comparison rules . When we create and modify a column definition, we can specify the character set and comparison rules of the column , The grammar is as follows ：

CREATE TABLE  Table name (
 Name   String type  [CHARACTER SET  Character set name ] [COLLATE  Comparison rule name ],
 Other columns ...
);

ALTER TABLE  Table name  MODIFY  Name   String type  [CHARACTER SET  Character set name ] [COLLATE  Comparison rule name ];

For a column , If the character set and comparison rules are not specified in the created and modified statements , The column of the table will be used
And comparison rule as the character set and comparison rule of the column .
Tips ：
When converting the character set of a column, you need to pay attention to , If the data stored in the front of conversion cannot be represented by the converted character set, an error will occur . For example, the character set used by the original column is utf8, Some Chinese characters are stored in the column , Now convert the character set of the column to ascii That would make a mistake , because ascii The character set cannot represent Chinese characters .

5.2.5. Summary

We introduce this 4 The relationship between level character sets and comparison rules is as follows ：

1. If Create or modify Columns There are no explicit character sets and comparison rules specified when , Then this column The default table is Character sets and comparison rules .
2. If Create table time There are no explicit character sets and comparison rules , Then the table By default, the database is used Character sets and comparison rules .
3. If When creating a database There are no explicit character sets and comparison rules , Then the database By default, the server's name is used Character sets and comparison rules .
   Knowing these rules , For a given table , We should know the character set and comparison rules of its columns , Thus, the storage space occupied by the actual data of each column is determined according to the type of this column .

For example, columns col The character set used is gbk , A character ‘ I ’ stay gbk The code in is 0xCED2 , Takes two bytes , The actual data of two characters takes up 4 Bytes . If you change the character set of this column to utf8 Words , These two characters actually occupy 6 Bytes .

5.3. Character sets and comparison rules ( understand )

5.3.1. utf8 And utf8mb4

utf8 The character set indicates that a character needs to use 1～4 Bytes , But some of the characters we often use 1～3 It's just a byte . The character set represents the maximum byte length of a character , In some aspects, it will affect the storage and performance of the system , So design MySQL The designer secretly defined two concepts ：

1. utf8mb3 ： Castrated utf8 Character set , Use only 1～3 Bytes represent characters .
2. utf8mb4 ： authentic utf8 Character set , Use 1～4 Bytes represent characters .

5.3.2. Compare the rules

The above table ,MySQL The version supports 41 A character set , Among them Default collation Columns represent a default comparison rule in this character set , It contains the language in which the comparison rule mainly works , such as utf8_polish_ci It means to compare the rules in Polish , utf8_spanish_ci It's a comparison of Spanish rules , utf8_general_ci It's a general rule of comparison .
The suffix indicates whether the comparison rule distinguishes accents in the language 、 Case write . As follows ：

suffix	English interpretation	describe
_ai	accent insensitive	Stress insensitive
_as	accent sensitive	Distinguish between stress
_ci	case insensitive	Case insensitive
_cs	case sensitive	Case sensitive
_bin	binary	Compare in binary terms

The last column Maxlen , It represents this kind of character set, indicating that a character needs up to a few bytes .
Common operations 1：

# see GBK Comparison rules for character sets 
SHOW COLLATION LIKE '%gbk%';
# see UTF-8 Comparison rules for character sets 
SHOW COLLATION LIKE '%utf8%';

Common operations 2：

# View the character set and comparison rules of the server 
SHOW VARIABLES LIKE '%_server';
# View the character set and comparison rules of the database 
SHOW VARIABLES LIKE '%_database';
# View the character set of the specific database 
SHOW CREATE DATABASE dbtest1;
# Modify the character set of the specific database 
ALTER DATABASE dbtest1 DEFAULT CHARACTER SET 'utf8' COLLATE 'utf8_general_ci';

Common operations 3：

# View the character set of the table 
show create database dbtest1;

# View table comparison rules 
show create table emp1;

# Modify the character set and comparison rules of the table 
ALTER TABLE emp1 DEFAULT CHARACTER SET 'utf8' COLLATE 'utf8_general_ci';

5.4. The change of character set from request to response

System variables	describe
character_set_client	The character set used by the server to decode the request
character_set_connection	When the server processes the request, it will change the request string from character_set_client To character_set_connection
character_set_results	The character set used by the server to return data to the client

The default values of these system variables on my computer are as follows （ The default values may vary from operating system to operating system ）：

show variables like '%character%';

In order to reflect the changes of character set in the process of request processing , Here, the value of a system variable is specially modified ：

 set character_set_connection = gbk;

Now suppose that the request sent by our client is the following string ：

SELECT * FROM t WHERE s = ' I ';

In order to facilitate everyone to understand this process , We only analyze characters ‘ I ’ In this process, the character set is converted .
Now let's look at the changes in the character set from the request sent to the result returned ：

1. The character set used by the client to send the request
   Generally, the character set used by the client is consistent with the current operating system , Different operating systems may use different character sets , as follows ：

• class Unix The system uses utf8.
• Windows It uses gbk.
  When the client uses utf8 Character set , character ‘ I ’ The byte form in the request sent to the server is ：0xE68891

 Tips 
 If you're using visualization tools , such as navicat And so on. , These tools may use custom character sets 
 To encode the string sent to the server , Instead of using the default character set of the operating system 
（ So when learning, try to use the command line window ）.

2. The request received by the server from the client is actually a string of binary bytes , It will think that the character set used by this string of bytes is character_set_client , Then convert this string of bytes into character_set_connection Character set encoded character .
   Because on my computer character_set_client The value of is utf8 , First of all, according to utf8 Character set pair byte string 0xE68891 decode , The resulting string is ‘ I ’ , And then according to character_set_connection Represents the character set , That is to say gbk Encoding , The result is a byte string 0xCED2 .
3. Because of the watch t The column of col It's using gbk Character set , And character_set_connection Agreement , So go directly to the column and find the byte value is 0xCED2 The record of , Finally found a record .

 Tips 
 If a column uses a character set and character_set_connection If the character set represented is inconsistent , It needs to be done 
 A character set conversion .

4. In the record found in the previous step col Column is actually a byte string 0xCED2 , col The column is gbk Coded , So first we will use this byte string gbk decode , Get a string ‘ I ’ , Then use this string character_set_results Represents the character set , That is to say utf8 Encoding , Got a new byte string ：0xE68891 , Then send it to the client .
5. Because the character set used by the client is utf8 , So you can successfully 0xE68891 Interpret as character I , So that it can be displayed on our display , So we humans also understand the returned results .
   The summary diagram is as follows ：
   

6、 ... and . SQL Case specification

6.1. Windows and Linux Platform differences

stay SQL in , Keywords and function names are case insensitive , such as SELECT、WHERE、ORDER、GROUP BY Other key words , as well as ABS、MOD、ROUND、MAX Equal function name .
But in the SQL in , You still have to determine the case specification , Because in Linux and Windows In the environment , You may encounter different case problems . windows System default case insensitive , however linux The system is case sensitive .
Check it with the following command ：

• Windows Under the system ：

SHOW VARIABLES LIKE '%lower_case_table_names%;

• Linux Under the system ：
  
• lower_case_table_names Parameter value setting ：

1. The default is 0, Case sensitive .
2. Set up 1, Case insensitive . Create a table , All databases are stored on disk in lowercase form , about sql Statements are converted to lowercase to find tables and databases .
3. Set up 2, The created tables and databases are stored according to the statement format , All searches are made in lowercase .

• On two platforms SQL The difference between case and case, specifically ：

MySQL stay Linux Next database name 、 Table name 、 Name 、 This is the case rule for aliases ：
1、 Database name 、 Table name 、 The table alias 、 Variable names are strictly case sensitive ;
2、 keyword 、 The function name is in  SQL  Case insensitive in ;
3、 Name （ Or field name ） Alias with column （ Or field alias ） In all cases, case is ignored ;

notes ：MySQL stay Windows All cases are not case sensitive

6.2. Linux Set the case rule

When you want to make it case insensitive , To be in my.cnf This configuration file [mysqld] Add lower_case_table_names=1 , Then restart the server .
notes ：

1. However, you need to convert the original database and tables to lowercase before restarting the database instance , Otherwise, the database name will not be found .
2. This parameter applies to MySQL5.7. stay MySQL 8 It is forbidden to restart MySQL Service will lower_case_table_names Set to be different from initialization MySQL Service settings lower_case_table_names value . If you have to MySQL8 Set to case insensitive , The specific steps are: ：

1、 stop it MySQL service 
2、 Delete data directory , Delete immediately  /var/lib/mysql  Catalog 
3、 stay MySQL The configuration file （ /etc/my.cnf ） Add  lower_case_table_names=1
4、 start-up MySQL service 

6.3. SQL Write suggestions

If your variable name naming convention is not uniform , There may be errors . Here's a suggestion for naming conventions ：

1.  Keywords and function names are all capitalized ;
2.  Database name 、 Table name 、 Table alias 、 Field name 、 All fields, aliases, etc. are lowercase ;
3. SQL  The statement must end with a semicolon .

Database name 、 Table name and field name are in Linux MySQL The environment is case sensitive , Therefore, it is recommended that you unify the naming rules of these fields , For example, all in lowercase .
Although keywords and function names are in SQL Case insensitive in , That is, if it is lowercase, it can also execute . But at the same time, all keywords and function names are capitalized , In order to distinguish the database name 、 Table name 、 Field name .

7、 ... and . sql_mode Reasonable setting of

7.1. Loose mode vs Strict mode

1. Loose mode ：
   If the setting is loose mode , So when we insert data , Even if it's given the wrong data , It may also be accepted , And don't report mistakes .
   give an example ： When I create a table , One of the fields in this table is name, to name When setting the field type of char(10) , If I
   When inserting data , among name This field corresponds to a piece of data It's longer than 10 , for example ’1234567890abc’, Exceeds the set field length 10, Then there will be no mistake , And take the front 10 Save characters , That is to say, your data is stored as ’1234567890’, and ’abc’ There is no the . however , The data we gave is wrong , Because the field length is exceeded , But there was no mistake , also mysql Disposed of and accepted , This is the effect of the relaxed model .
   Application scenarios ： By setting sql mode For the relaxed mode , To ensure that most sql Conforming to the standard sql grammar , In this way, the application is carried out between different databases transfer when , There is no need for business sql Make big changes .
2. Strict mode ：
   The above loose mode error appears , It's right to report a mistake , therefore MySQL5.7 The version will sql_mode Default to strict mode . So in Production environment in , What we have to adopt is a strict model , , in turn, Development 、 Test environment Your database must also be set , In this way, problems can be found during the development and testing phase . And even if we use MySQL5.6, It should also be changed to a strict model .
   Development experience ：MySQL The database always wants to take care of all the operations related to data , Including data verification , Actually development
   in , We should be in ourselves The developed project program level makes these checks , Although there are some steps in writing the project , But after that , We are doing database migration or project migration , It will be much more convenient .
   The possible problems after changing to strict mode ：
   If the setup mode includes NO_ZERO_DATE , that MySQL Database is not allowed to insert zero date , Inserting a zero date throws an error instead of a warning . for example , Table with fields TIMESTAMP Column （ If it is not declared NULL Or display DEFAULT Clause ） Will automatically assign DEFAULT ‘0000-00-00 00:00:00’（ Zero time stamp ）, This is obviously not satisfied sql_mode Medium NO_ZERO_DATE And report wrong .

7.2. Mode viewing and setting

• View the current sql_mode

select @@session.sql_mode
select @@global.sql_mode
# perhaps 
show variables like 'sql_mode';

• Temporary setting mode ： Set the settings in the current window sql_mode：

SET GLOBAL sql_mode = 'modes...'; # overall situation 
SET SESSION sql_mode = 'modes...'; # Current session 

give an example ：

# Change to strict mode . This method only works in the current session , Closing the current session will not take effect .
set SESSION sql_mode='STRICT_TRANS_TABLES';
# Change to strict mode . This method works in the current service , restart MySQL Failure after service .
set GLOBAL sql_mode='STRICT_TRANS_TABLES';

• Permanent setting mode ： stay /etc/my.cnf Middle configuration sql_mode
  stay my.cnf file (windows System is my.ini file ), newly added ：

[mysqld]
sql_mode=ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR
_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION

then restart MySQL .
Of course, restart is forbidden in the production environment MySQL Service , So using Temporary setting mode + Permanent setting mode To solve online problems , So even if it's restarted one day MySQL service , It's going to be permanent .