A brief discussion on the matching rules of host and user when Mysql connects to the database

When multiple matches are possible, the server must determine which of them to use. It resolves this issue as follows: 
Whenever the server reads the user table into memory, it sorts the rows. 
When a client attempts to connect, the server looks through the rows in sorted order. 
The server uses the first row that matches the client host name and user name. 
The server uses sorting rules that order rows with the most-specific Host values ​​first. Literal host names and IP addresses are the most specific. (The specificity of a literal IP address is not affected by whether it has a netmask, so 198.51.100.13 and 198.51.100.0/255.255.255.0 are considered equally specific.) The pattern '%' means "any host" and is least specific. The empty string '' also means "any host" but sorts after '%'. Rows with the same Host value are ordered with the most-specific User values ​​first (a blank User value means "any user" and is least specific). For rows with equally-specific Host and User values, the order is nondeterministic.

mysql> SELECT authentication_string, host, user,account_locked FROM mysql.USER ORDER BY host desc ,user desc;
+-------------------------------------------+--------------+---------------+----------------+
| authentication_string | host | user | account_locked |
+-------------------------------------------+--------------+---------------+----------------+
| *511C0A408C5065XXEC90D60YYA1AB9437281AF28 | localhost | root | N |
| *THISISNOTAVALIXXASSWORDYYATCANBEUSEDHERE | localhost | mysql.sys | Y |
| *THISISNOTAVALIXXASSWORDYYATCANBEUSEDHERE | localhost | mysql.session | Y |
| *485CE31BA547A4XXC047659YY10DF200F361CD4E | localhost | bkpuser | N |
| *7B502777D8FF69XX4B56BC2YY2867F4B47321BA8 | 192.168.56.% | repl | N |
| *AECCE73463829AXX3968838YYF6F85E43C3F169C | % | flyremote | N |
| *566AC8467DAAAEXXE247AE7YY0A770E9B97D9FB0 | | flylocal | N |
+-------------------------------------------+--------------+---------------+----------------+
8 rows in set (0.00 sec)
 

mysql> create user ''@'localhost' identified by "Kong123$";
Query OK, 0 rows affected (0.00 sec) 
mysql> create user ''@'' identified by "doubleKong123$";   
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT authentication_string, host, user,account_locked FROM mysql.USER ORDER BY host desc ,user desc;
+-------------------------------------------+--------------+---------------+----------------+
| authentication_string | host | user | account_locked |
+-------------------------------------------+--------------+---------------+----------------+
| *511C0VVV8C5065CBEC90D6TTTT1AB9437281AF28 | localhost | root | N |
| *THISIVVVTAVALIDPASSWORTTTTTCANBEUSEDHERE | localhost | mysql.sys | Y |
| *THISIVVVTAVALIDPASSWORTTTTTCANBEUSEDHERE | localhost | mysql.session | Y |
| *485CEVVVA547A48CC04765TTTT0DF200F361CD4E | localhost | bkpuser | N |
| *256D7VVV91F7363EBDADEFTTTTB74B2B318746FC | localhost | | N |
| *7B502VVVD8FF69164B56BCTTTT867F4B47321BA8 | 192.168.56.% | repl | N |
| *AECCEVVV63829A5F396883TTTT6F85E43C3F169C | % | flyremote | N |
| *566ACVVV7DAAAE79E247AETTTTA770E9B97D9FB0 | | flylocal | N |
| *AE162VVV68403D1D98A4C9TTTT50A508B8C56F3F | | | N |
+-------------------------------------------+--------------+---------------+----------------+
9 rows in set (0.00 sec)

[root@hostmysql-m mysql]# mysql -uflyremote -pFlyremote123$
mysql: [Warning] Using a password on the command line interface can be insecure.
ERROR 1045 (28000): Access denied for user 'flyremote'@'localhost' (using password: YES)

[root@hostmysql-m mysql]# mysql -uflyremote -pKong123$
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 15
Server version: 5.7.23-log MySQL Community Server (GPL) 
Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved. 
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners. 
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> select user(),CURRENT_USER();
+---------------------+----------------+
| user() | CURRENT_USER() |
+---------------------+----------------+
| flyremote@localhost | @localhost |
+---------------------+----------------+
1 row in set (0.06 sec)

[root@hostmysql-m mysql]# mysql -uflyremote -pFlyremote123$ -h127.11.22.33 
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 12
Server version: 5.7.23-log MySQL Community Server (GPL) 
Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved. 
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners. 
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement. 
mysql>

mysql> select user(),CURRENT_USER();
+------------------------+----------------+
| user() | CURRENT_USER() |
+------------------------+----------------+
| [email protected] | flyremote@% |
+------------------------+----------------+
1 row in set (0.00 sec)

[root@hostmysql-m ~]# mysql -uhahaha -pdoubleKong123$ -h127.11.22.33
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 6
Server version: 5.7.23-log MySQL Community Server (GPL) 
Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved. 
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners. 
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement. 
mysql>

mysql> select user(),CURRENT_USER();
+---------------------+----------------+
| user() | CURRENT_USER() |
+---------------------+----------------+
| [email protected] | @ |
+---------------------+----------------+
1 row in set (0.01 sec)

This program enables you to improve the security of your MySQL installation in the following ways:
 You can set a password for root accounts.
 You can remove root accounts that are accessible from outside the local host.
 You can remove anonymous-user accounts.
 You can remove the test database (which by default can be accessed by all users, even anonymous users), and privileges that permit anyone to access databases with names that start with test_.

 //Remove anonymous users
 remove_anonymous_users(); 
/**
 Removes all the anonymous users for better security.
*/
void remove_anonymous_users()
{
 int reply;
 reply = get_response((const char *) "By default, a MySQL installation has an "
      "anonymous user,\nallowing anyone to log "
      "into MySQL without having to have\na user"
      "account created for them. This is intended "
      "only for\ntesting, and to make the "
      "installation go a bit smoother.\nYou should"
      "remove them before moving into a production\n"
      "environment.\n\nRemove anonymous users? "
      "(Press y|Y for Yes, any other key for No) : ", 'y');
 
 if (reply == (int) 'y' || reply == (int) 'Y')
 {
 const char *query;
 query = "SELECT USER, HOST FROM mysql.user WHERE USER = ''";
 if (!execute_query(&query, strlen(query)))
  DBUG_PRINT("info", ("query success!"));
 MYSQL_RES *result = mysql_store_result(&mysql);
 if (result)
  drop_users(result);
 mysql_free_result(result);
 fprintf(stdout, "Success.\n\n");
 }
 else
 fprintf(stdout, "\n ... skipping.\n\n");
}

| root | localhost | *81F5E21E35407D884A6CD4A731AEBFB6AF209E1B |
| root | % | *81F5E21E35407D884A6CD4A731AEBFB6AF209E1B |