Mysql database related operations

Hello everyone , This time I bring you my new column ： database , The database is relatively not that difficult , As long as you practice hard 、 Memorize grammar , I believe that learning database well is not a problem , Bloggers will start from 0 analyse , Explain the knowledge points of database step by step , And will give many examples . most important of all , Bloggers will not use software , Use the most primitive way -> Command line to explain , The advantage of this explanation is to write sentence by sentence , It won't cause the problem that readers can't keep up with their ideas ！
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B standing ： Under construction , I will consider in the future B Explain some knowledge points on the station and so on

1、 Operation of database

1.1 Displays the current database

Displays which databases exist on the current server ( Or what data sets )

grammar ：

show databases;

Be careful ：

1. databases It's the plural
2. show and databases Space between ( A few don't matter , But not without ！)
3. SQL Statements are basically based on ; At the end of the
4. SQL Statement is case insensitive
5. The default built-in database , Don't move about ！ It's not fun to break the database ~

1.2 Create database

grammar ：

create database [if not exists]  Database name  [ Create rules ];

Be careful ：

1. All the rest [] There is no need to input , This [] Represents optional ( Write but not write );

2. As long as the name of the database is not the name of the keyword ( In fact, you can also create databases with keywords as their names , Use inverted quotation marks , namely Just wrap this symbol ( Position in Tab upper ,1 To the left of ,Esc Below )）, But be careful : Not on a server Create multiple databases with the same name

   

3. if not exists The function of is to create as long as it doesn't exist , In fact, it can be

   Use scenarios : You send someone a paragraph SQL, But you don't know whether there is a corresponding database on his computer , So choose if not to create this way . Others have already created , Let's not fool around

4. Inside database It's singular

5. Create rules ： When creating a database, you can specify character sets and verification rules

   1. Validation rules : Describes the comparison method between current characters

      Compare the equality of strings / Size relationship ( For example, we sometimes want to ignore case )

      There are not many scenarios , We seldom intervene manually , Generally, you can use the default verification rules .

   2. Character set :

      create database test character set utf8mb4;
      

      

      Inside utf8mb4 What's going on ？

      First , Let's think about it first , How does a computer save letters ？

      ASCII code ： By numbers , To represent letters . The one-to-one correspondence between numbers and letters , become ASCII clock

      however ASCII It does not contain Chinese , So now two versions are commonly used

      1. GBK

         2 Bytes to represent a Chinese character

         yes Windows The default character encoding of simplified Chinese version

      2. UTF-8: Be regarded as " Universal character encoding "

         UTF8 Variable length coding : A Chinese character , In a few bytes , It's uncertain .

         It's usually 3 A byte means a Chinese character

      Essentially , Is to use numbers to represent symbols in various languages , Corresponding large table , This mapping relationship is called " Character set "

      So the one above utf8mb4 What's going on ？

      utf8mb4 Actually, it's better than utf8 There are some special symbols , such as emoji expression . Set to utf8 Nothing

      So we learned such a thing before

      char* s = " hi ";
      printf("%d\n",strlen(s));
      
      // So, please. , How many bytes does a Chinese character occupy 
      // Through the right GBK as well as UTF8 Explanation , We can see :
      // You need to confirm which character code it is 
      // GBK -> 2 Bytes 
      // UTF8 -> 3 Bytes 1
      

6. Implement this SQL Time spent in operation ： We can see the picture just now , The running time is 0.03sec, This is relative to computers , It's actually a very slow speed

   Let me introduce you the following time of common computer operations ：

   Access the network ：s~ms

   Access disk ：ms~us

   Access memory ：us~ns

   operation CPU：ps

   Be careful ： This conversion is 1000, No 1024. Don't get mixed up

   Databases are often slow links , Large projects are prone to problems .

1.3 Using a database

Before performing a series of operations on the database , We must select the database first

grammar ：

use  Database name ;

Use the database statement , No addition ; Can also carry out . But it's not recommended , stay SQL in , Some sentences can be omitted ;, But some statements must be added ;, So we add ;, Let you see which SQL The statement did not add ; Don't be surprised

1.4 Delete database

This operation is dangerous ！！！ Never delete the database on the company's server .

grammar :

drop database  Database name ;

In fact, it can be retrieved , But it's troublesome .

Can be opened MySQL Of binlog function , Every operation on the database , Will be recorded as binlog in . ten thousand ⼀ Data destroyed , You can re binlog Import all operations in ⼀ All over , But the content is not complete

So is there a way , Avoid deleting data in the production environment ？

1. Access control ： Provide administrator privileges , Only administrators can delete

   Produced a profession : DBA Database administrator

2. Back up a copy ： Once deleted , There is still a way to go

   Is it possible that permission control has also failed , The backups have also been deleted ？ It's certainly possible , That is not “ False deletion ” 了 , It was intentional , Will be sent to a good place to eat and live .

3. Recover hard disk data :

   

1.5 Expand ：cmd Minor operations

1. Press and hold the up and down keys on the keyboard to find historical commands （ Save your energy ）
2. Copy and paste ： Select content , Press enter to copy , Right click Paste

1.6 The environment involved in the company ( Digression )

1. Office environment ： On entry , The computer sent to you by the company ( Of course, there are also a few companies , I will let you bring your computer -> Basically run fast , Far away ), Will use this computer for daily office . The basic configuration is 8 nucleus CPU 16G Memory 512G Hard disk

2. development environment ： It is generally provided in the form of server , The office computer is connected to the server through the network . The corresponding device of this server is generally about 16 nucleus CPU 64G Memory 4T Hard disk

   Some projects are very resource consuming , A bad start-up memory will eat dozens G, Or even hundreds G. Of course, you can't use your own office computer .

   If the developed program is relatively small , At this time, there may be no development environment , You can directly use the office computer to develop .

   Generally, the company brings the equipment that has passed the warranty period to the programmer as " development environment "

3. Test environment : The environment used by the testing staff for the program to be tested , The machines used are basically those that have passed the warranty period

4. The online environment / Production environment ： It is also provided in the form of server , You also need to connect remotely through the network of your office computer . Good configuration , about 56 nucleus CPU 256G/512G Memory A number of T Hard disk

   Production environment , In fact, it is an environment that provides services to external users , Never hang up ！

   Hanging up will cause major resource losses

that MySQL The server will basically appear on the top 2 3 4 Environment , If it is deleted by mistake 2 3 Database in the environment , In fact, the situation is relatively good , The colleagues worked overtime and were rescued , Basically, it will be scolded by many people . But once you accidentally delete 4 That is, the database in the online environment , Then it's over . Users outside will feel the direct impact , The probability will lead to the loss of users , Take legal action and so on . Serious will cause major safety accidents !