MySQL如何修改用户密码以及如何重置root密码

a、可以在创建用户的时候指定密码，以及直接使用grant创建用户的时候指定密码。
 对于已经存在的用户直接使用grant方式也可以修改密码
如下：
--演示版本
root@localhost[(none)]> show variables like 'version%'; 
+-------------------------+------------------------------+ 
| Variable_name | Value | 
+-------------------------+------------------------------+ 
| version | 5.5.37 | 
| version_comment | MySQL Community Server (GPL) | 
| version_compile_machine | x86_64 | 
| version_compile_os | Linux | 
+-------------------------+------------------------------+ 
--下面我们使用grant方式创建一个新帐户fred，并设定密码
root@localhost[(none)]> grant usage on *.* to 'fred'@'localhost' identified by 'fred';
Query OK, 0 rows affected (0.00 sec)
--查看刚刚创建的账户
root@localhost[(none)]> select host,user,password from mysql.user where user='fred';
+-----------+------+-------------------------------------------+
| host | user | password |
+-----------+------+-------------------------------------------+
| localhost | fred | *6C69D17939B2C1D04E17A96F9B29B284832979B7 |
+-----------+------+-------------------------------------------+
--下面可以成功登陆mysql
SZDB:~ # mysql -ufred -pfred
fred@localhost[(none)]> 
b、使用set password方式来修改账户密码
--下面我们使用set password方式来设定密码
root@localhost[(none)]> set password for 'fred'@'localhost'=password('passwd');
Query OK, 0 rows affected (0.00 sec)
root@localhost[(none)]> flush privileges;
Query OK, 0 rows affected (0.00 sec)
--再次登陆时，之前的密码已经失效，无法登陆
SZDB:~ # mysql -ufred -pfred
ERROR 1045 (28000): Access denied for user 'fred'@'localhost' (using password: YES)
--下面使用新密码登陆成功
SZDB:~ # mysql -ufred -ppasswd
fred@localhost[(none)]> 
--检索数据库是否存在jack用户，如下密码为null
root@localhost[(none)]> select host,user,password from mysql.user where user='jack';
+-----------+------+----------+
| host | user | password |
+-----------+------+----------+
| localhost | jack | |
+-----------+------+----------+
c、加密方式更新系统表user的password列
--我们尝试直接更新密码列(不使用加密函数方式)
root@localhost[(none)]> update mysql.user set password='jack' where user='jack';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
--由于直接使用明文，因此系统表user列password显示为明文
root@localhost[(none)]> select host,user,password from mysql.user where user='jack';
+-----------+------+----------+
| host | user | password |
+-----------+------+----------+
| localhost | jack | jack |
+-----------+------+----------+
--Author : Leshami
--Blog ：http://blog.csdn.net/leshami
root@localhost[(none)]> flush privileges;
Query OK, 0 rows affected (0.02 sec)
--此时无法登陆
SZDB:~ # mysql -ujack -pjack -h localhost 
ERROR 1045 (28000): Access denied for user 'jack'@'localhost' (using password: YES)
--下面我们通过set方式来修改jack的密码，提示找不到jack用户
root@localhost[(none)]> set password for 'jack'@'localhost'=password('jack');
ERROR 1133 (42000): Can't find any matching row in the user table
--我们切换到mysql数据库下尝试，
root@localhost[(none)]> use mysql 
root@localhost[mysql]> set password for 'jack'@'localhost'=password('passwd'); 
--在mysql数据库下依旧无法更新用户jack的密码
ERROR 1133 (42000): Can't find any matching row in the user table
--下面我们尝试用password函数方式来更新password列
root@localhost[mysql]> update user set password=password('passwd') where user='jack'; --此方式更新成功
Query OK, 1 row affected (0.04 sec)
Rows matched: 1 Changed: 1 Warnings: 0
root@localhost[mysql]> select host,user,password from user where user='jack'; --可以看到密码已经变成了密文
+-----------+------+-------------------------------------------+
| host | user | password |
+-----------+------+-------------------------------------------+
| localhost | jack | *59C70DA2F3E3A5BDF46B68F5C8B8F25762BCCEF0 |
+-----------+------+-------------------------------------------+
root@localhost[mysql]> flush privileges;
Query OK, 0 rows affected (0.00 sec)
--此时登陆成功
robin@SZDB:~> mysql -ujack -ppasswd 
jack@localhost[(none)]> 

--假定此时我们的root帐户忘记或遗失了密码，如下面的演示，我们给出的是xxx，不能登陆到mysql(真实的密码为mysql)
SZDB:~ # mysql -uroot -pmysql
root@localhost[(none)]> 
SZDB:~ # mysql -uroot -pxxx #忘记密码，此时无法正常登录 
Enter password: 
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)
--首先停止mysql服务器
SZDB:~ # service mysql stop
Shutting down MySQL.. done
--使用--skip-grant-tables选项跳过授权表验证，
SZDB:~ # mysqld --help --verbose #获取mysqld帮助信息
--skip-grant-tables Start without grant tables. This gives all users FULL
 ACCESS to all tables.
--使用--skip-grant-tables启动mysql服务器
SZDB:~ # mysqld --skip-grant-tables --user=mysql &
[1] 10209
SZDB:~ # ps -ef | grep mysql
mysql 10209 14240 4 13:52 pts/0 00:00:00 mysqld --skip-grant-tables --user=mysql
root 10229 14240 0 13:53 pts/0 00:00:00 grep mysql
SZDB:~ # mysql 
root@localhost[(none)]> select user,host,password from mysql.user where user='root';
+-------+-----------+-------------------------------------------+
| user | host | password |
+-------+-----------+-------------------------------------------+
| root | % | *E74858DB86EBA20BC33D0AECAE8A8108C56B17FA |
| root | 127.0.0.1 | *E74858DB86EBA20BC33D0AECAE8A8108C56B17FA |
+-------+-----------+-------------------------------------------+
--更新mysql账户密码为NULL或设定为新密码，注设定为空密码时可以直接设置，无须使用加密函数，2者等同
root@localhost[(none)]> update mysql.user set password='' where user='root';
Query OK, 2 rows affected (0.00 sec)
Rows matched: 2 Changed: 2 Warnings: 0
root@localhost[(none)]> select user,host,password from mysql.user where user='root';
+------+-----------+----------+
| user | host | password |
+------+-----------+----------+
| root | % | |
| root | 127.0.0.1 | |
+------+-----------+----------+
root@localhost[(none)]> exit
Bye
#再次停止mysql数据库服务器
SZDB:~ # service mysql stop
Shutting down MySQL. done
[1]+ Done mysqld --skip-grant-tables --user=mysql
SZDB:~ # service mysql start
Starting MySQL.. done
SZDB:~ # mysql #重启后再次登陆，不再需要任何密码
root@localhost[(none)]>