修改自己的密码（root用户，其它用户应该也差不多）
方法一：
[root@localhost /]# mysqladmin -u root -p password "root" #修改密码为root
Enter password: #输入旧密码
[root@localhost /]# mysql -uroot -p #尝试使用旧密码登录
Enter password:
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: YES)
[root@localhost /]# mysql -uroot -p #输入新密码root登录
Enter password:
Welcome to the MariaDB monitor. Commands end with ; or \g.
Your MariaDB connection id is 19
Server version: 5.5.52-MariaDB MariaDB Server
Copyright (c) 2000, 2016, Oracle, MariaDB Corporation Ab and others.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
MariaDB [(none)]>
方法二：
在mysql.user中使用update更新密码
方法三：
或者进入mysql后，使用set修改密码
[root@localhost /]# mysql -uroot -p #使用旧密码root登录
Enter password:
Welcome to the MariaDB monitor. Commands end with ; or \g.
Your MariaDB connection id is 19
Server version: 5.5.52-MariaDB MariaDB Server
Copyright (c) 2000, 2016, Oracle, MariaDB Corporation Ab and others.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
MariaDB [(none)]> set password=password("123456"); #修改密码为123456，我一直很好奇为什么密码必须用password扩起来，后来知道了，新密码必须用password来加密
Query OK, 0 rows affected (0.00 sec)
MariaDB [(none)]> quit
Bye
[root@localhost /]# mysql -uroot -p #使用新密码123456登录
Enter password:
Welcome to the MariaDB monitor. Commands end with ; or \g.
Your MariaDB connection id is 20
Server version: 5.5.52-MariaDB MariaDB Server
Copyright (c) 2000, 2016, Oracle, MariaDB Corporation Ab and others.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
MariaDB [(none)]>
root用户修改指定用户密码
方法一
MariaDB [(none)]> set password for 'bp'@'localhost'=password("123456");
Query OK, 0 rows affected (0.01 sec)
方法二：
MariaDB [(none)]> update mysql.user set password=password("123") where user='bp' and host='localhost'; #使用update修改密码，修改成功后，我打开另一个终端使用该用户登录数据库，发现无法使用新密码登录，但使用旧密码可以登录
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
MariaDB [(none)]> select user,host from mysql.user; #因为报错信息里面包含localhost，于是查看用户表信息有没有错，遗憾的是没有
+---------+-----------------------+
| user | host |
+---------+-----------------------+
| aa | % |
| aaa | % |
| root | 127.0.0.1 |
| root | ::1 |
| | localhost |
| aa | localhost |
| bb | localhost |
| bp | localhost |
| ggo | localhost |
| my | localhost |
| mytest | localhost |
| newuser | localhost |
| nome | localhost |
| root | localhost |
| | localhost.localdomain |
| root | localhost.localdomain |
+---------+-----------------------+
16 rows in set (0.00 sec)
MariaDB [(none)]> flush privileges; #后来想起来，是不是还要刷新权限。刷新之后，使用新密码可以登录
Query OK, 0 rows affected (0.00 sec)
方法三：grant修改密码
MariaDB [mytest]> grant select on mytest.test to 'bp'@'localhost' identified by 'linux';
Query OK, 0 rows affected (0.05 sec) #这个不需要刷新权限。。
mysql5.7修改密码
cat /var/log/mysqld.log|grep 'temporary password'
alter user 'root'@'localhost' identified by 'root';
忘记密码（需要重启服务器）
在/etc/my.cnf的mysqld里面增加skip-grant-tables （5.7以前的应该是skip-grant）
重启mysqld
mysql> update mysql.user set authentication_string=password(',,,abc123...') where user='root'; （旧版的应该是update mysql.user set password=password(',,,abc123...') where user='root';）
Query OK, 1 row affected, 1 warning (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 1
mysql> flush privileges;
Query OK, 0 rows affected (0.00 sec)
mysql> quit
重启服务器