MySQL advanced (XVII) cannot connect to database server problem analysis

One 、 Preface

Because the database user name and password used in different projects are inconsistent , During the implementation of an earlier project “

The user specified as a definer ('root'@'localhost') does not exist

” Error prompt for . After a search on the Internet , as a result of root The user doesn't exist , So I changed my user name back to the previous user name , As a result, the database cannot be connected , As shown in the figure below :

 

Two 、 Problem analysis

• Maybe the database service is not started ;

• The specified database connection user name or password may be incorrect ;

• Maybe the database connection statement is written incorrectly ;

• Maybe no specified database exists in the database ;

• Probably not sql Package or no driver ;

3、 ... and 、 Solution

Mysqlbin Catalog : 

C:\Program Files\MySQL\MySQL Server 5.7\bin

mysql -u root -p;

After deleting the original database instance , Create a new instance , It is found that the database cannot be connected , As shown below :

 

Shut down and restart mysql service , Found or unable to log in , I feel bored .

 

take mysql After the service is shut down , Can enter the workbench, But still can't connect mysql. According to the figure below mysql The configuration file

my.ini

Location :

 

Can't connect to MySQL server on '127.0.0.1' (10061)

Check that the password is correct .

If the password is wrong, it will prompt that the password is wrong ,

Take a look at your hosts The file of .

Hosts file location :

C:\Windows\System32\drivers\etc

  Is it the port problem ？ So use

telnet 127.0.0.1 3308

Test port , It turned out to suggest ：

 

my.ini

What is really practical is 3308 port , use

netstat -an

see ,3308 In the following

TIME_WAIT

Overtime

state :

 

What's the reason for that ？ The test findings in the project are also hints

“Host 'localhost' is not allowed to connect to this MySQL server”

！

start-up

mysql

service , The following error appears :

 

After a fruitless attempt , reinstall ！

During the unloading process, because it was not unloaded completely , This leads to a series of problems during installation . tragedy ！

Mysql Uninstall the tutorial :

Click to see

Mysql Installation tutorial :

  

Windows Next MySQL 5.6 Detailed diagram of installation and configuration -51CTO.COM

The following error occurred during installation :

 

Various methods of network search , Still failed to solve the problem .

Change your mind again , Try another version , Which tried 5.7.10. But it is zip Format , After decompression, you need to configure a series of files , After a fruitless attempt , Used a lower version 5.5.47,msi Format , This is a fool installation method or more convenient . After the first installation , I have tried to restore the previous database , To die without fruit , And the user name cannot be changed , Another crazy unloading , install , After the second time , The user name can be changed normally . After testing , Everything is all right ！

The database should be backed up in time , Otherwise, the consequences are unimaginable ！

​ Four 、 Extended reading  mysql Batch deletion of large amounts of data

Suppose there is a table (syslogs) Yes 1000 Ten thousand records , You need to delete it without stopping business statusid=1 All records , Is almost 600 Ten thousand ,  Direct execution

 DELETE FROM syslogs WHERE statusid=1

  You will find that the deletion failed , because

lock wait timeout exceed

Error of .

Because this statement involves too many records , So we passed LIMIT Parameters are deleted in batches , Like every 10000 Delete once , Then we can use  

MySQL

It's a statement like this :

DELETE FROM syslogs WHERE status=1 ORDER BY statusid LIMIT 10000;

Then these records can be successfully deleted by executing them several times .

When performing mass deletion, pay attention to the use of

limit

. Because if not

limit

, Deleting a large amount of data is likely to cause deadlock . If

delete

Of

where

Statement is not on Index , You can find the primary key first , Then delete the database according to the primary key . At ordinary times

update

and

delete

It's better to add

limit 1

To prevent misoperation .