Currently used user and host:

mysql> select USER();

+----------------+

| USER()         |

+----------------+

| root@localhost |

+----------------+

1 row in set (0.00 sec)

Add user

Before mysql 5, you can directly use INSERT to INSERT mysql users into mysql tables. You can't do this after mysql 5  

mysql> insert into mysql.user(Host,User,Password) values('localhost','test_user',password('123123'));

ERROR 1062 (23000): Duplicate entry 'localhost-test_user' for key 'PRIMARY'

Add the user {GRANT the user the specified data table permission [use GRANT command to authorize the user accordingly]}

mysql> GRANT all privileges ON table1.* TO 
'test_user'@'localhost' IDENTIFIED BY '123123' WITH GRANT OPTION;

Query OK, 0 rows affected (0.02 sec)

IDENTIFIED BY specifies the user's login password

ALL PRIVILEGES refers to all permissions. You can also use select, update and other permissions  
*The * sign in front of the. \ is used to specify the database name, and the * sign in the back is used to specify the table name  
TO means TO give permission TO a user  
ON is used to specify which libraries and tables permissions are targeted  
'test Uuser' @'localhost 'means test Uuser, followed by @ by restricted host, which can be IP, IP segment, domain name and%,% means anywhere  
WITH GRANT OPTION this option means that the user can authorize his own permissions to others
 
You need to refresh the system permission table [flush privilege] for the user to log in effectively

mysql> flush privileges;    

delete user

mysql> drop user 'test_user'@'localhost';

View the permissions of the current user

mysql> SHOW GRANTS;
+----------------------------------------------------------------------------------------------------------------------------------------+
| Grants for root@localhost                                                                                                              |
+----------------------------------------------------------------------------------------------------------------------------------------+
| GRANT ALL PRIVILEGES ON *.* TO 'root'@'localhost' IDENTIFIED BY PASSWORD '\*E56A114692FE0DE073F9A1DD68A00EEB9703F3F1' WITH GRANT OPTION |
| GRANT PROXY ON ''@'' TO 'root'@'localhost' WITH GRANT OPTION                                                                           |
+----------------------------------------------------------------------------------------------------------------------------------------+

View the permissions of a user

mysql> show grants for 'test_user'@'localhost'
+------------------------------------------------------------------------------------------------------------+
| Grants for test_user@localhost                                                                                   |
+------------------------------------------------------------------------------------------------------------+
| GRANT USAGE ON *.* TO 'test_user'@'localhost' IDENTIFIED BY PASSWORD '\*E56A114692FE0DE073F9A1DD68A00EEB9703F3F1' |
| GRANT ALL PRIVILEGES ON table1.* TO 'test_user'@'localhost' WITH GRANT OPTION                                 |
+------------------------------------------------------------------------------------------------------------+

Rename account

mysql> rename user 'test_user'@'localhost' to 'bb'@'localhost';

Change Password

1. Use the set password command  

mysql> SET PASSWORD FOR 'test_user'@'localhost' = PASSWORD('123456');

2. Use mysqladmin [enter bin directory]   
Note: {format: mysqladmin -u user name - p old password new password]

/usr/bin$ mysqladmin -utest_user -p123456 password 123123
mysqladmin: Can't turn off logging; error: 'Access denied; you need (at least one of) the SUPER privilege(s) for this operation'

3. Directly edit the user table with update  

mysql> use mysql
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql> update user set PASSWORD = PASSWORD('123123') where user = 'test_user';
Query OK, 1 row affected (0.04 sec)
Rows matched: 1  Changed: 1  Warnings: 0