案例环境：
操作系统 ：Red Hat Enterprise Linux Server release 5.7 (Tikanga) 64 bit
数据库版本 : Mysql 5.6.19 64 bit
案例介绍：
今天开始学习mysql，遂先安装了Mysql 5.6.19 64bit 版本的数据库，结果安装成功了，但是使用root登录时遇到了ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)错误. 如下所示
[root@DB-Server tmp]# rpm -ivh MySQL-server-5.6.19-1.rhel5.x86_64.rpm
Preparing... ########################################### [100%]
1:MySQL-server ########################################### [100%]
[root@DB-Server tmp]# rmp -ivh MySQL-client-5.6.19-1.rhel5.x86_64.rpm
-bash: rmp: command not found
[root@DB-Server tmp]# rpm -ivh MySQL-client-5.6.19-1.rhel5.x86_64.rpm
Preparing... ########################################### [100%]
1:MySQL-client ########################################### [100%]
[root@DB-Server tmp]# /etc/rc.d/init.d/mysql status
MySQL is not running[FAILED]
[root@DB-Server tmp]# /etc/rc.d/init.d/mysql start
Starting MySQL.[ OK ]
[root@DB-Server tmp]# mysql -u root -p
Enter password:
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)
[root@DB-Server tmp]# mysql -u root mysql
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)
安装过程中没有设置过root密码，不清楚root密码，搜索了网上一些资料，大致有下面一些情况：
1：有些博客说root的随机密码位于/root/.mysql_secret中，但是我这个版本根本没有/root/.mysql_secret文件。搜索到一篇文章，大意如下所示（没有确认）。到现在也没有明白，我这个版本为什么没有生成这个文件？
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO) :表示没有生成root的临时密码
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: YES) :表示生成了root的临时密码。
2：有些资料说root的默认密码为空，经过确认那是以前的老版本，Mysql 5.6及以后版本出处于安全考虑，root密码已经不为空了。
另外，我在官网论坛居然发现有很多人遇到类似问题ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)
解决方法：
首先停掉mysql服务器
[root@DB-Server init.d]# /etc/rc.d/init.d/mysql stop
Shutting down MySQL..[ OK ]
然后使用mysqld_safe命令在启动mysql，更新root账号的密码
--skip-grant-tables：不启动grant-tables(授权表),跳过权限控制。
--skip-networking ：跳过TCP/IP协议，只在本机访问(从网上有些资料看，这个选项不是必须的。可以不用)
[root@DB-Server init.d]# mysqld_safe --user=mysql --skip-grant-tables --skip-networking &
[2] 9396
[root@DB-Server init.d]# 140722 14:59:46 mysqld_safe Logging to '/var/lib/mysql/DB-Server.err'.
140722 14:59:46 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql
执行上面命令后，此会话窗口会出现无反应的状态，需要使用CTRL+C中断会话，检查/var/lib/mysql/DB-Server.err日志，发现也无其它异常信息。没有弄明白该会话窗口为啥会出现无响应状态。
tail -200 /var/lib/mysql/DB-Server.err | more
2014-07-22 14:59:41 9346 [Note] Shutting down plugin 'binlog'
2014-07-22 14:59:41 9346 [Note] /usr/sbin/mysqld: Shutdown complete
140722 14:59:41 mysqld_safe mysqld from pid file /var/lib/mysql/DB-Server.pid ended
140722 14:59:46 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql
2014-07-22 14:59:47 0 [Warning] TIMESTAMP with implicit DEFAULT value is deprecated. Please use --explicit_defaults_for_timesta
mp server option (see documentation for more details).
2014-07-22 14:59:47 9516 [Note] Plugin 'FEDERATED' is disabled.
2014-07-22 14:59:47 9516 [Note] InnoDB: Using atomics to ref count buffer pool pages
2014-07-22 14:59:47 9516 [Note] InnoDB: The InnoDB memory heap is disabled
2014-07-22 14:59:47 9516 [Note] InnoDB: Mutexes and rw_locks use GCC atomic builtins
2014-07-22 14:59:47 9516 [Note] InnoDB: Compressed tables use zlib 1.2.3
2014-07-22 14:59:47 9516 [Note] InnoDB: Using Linux native AIO
2014-07-22 14:59:47 9516 [Note] InnoDB: Using CPU crc32 instructions
2014-07-22 14:59:47 9516 [Note] InnoDB: Initializing buffer pool, size = 128.0M
2014-07-22 14:59:47 9516 [Note] InnoDB: Completed initialization of buffer pool
2014-07-22 14:59:47 9516 [Note] InnoDB: Highest supported file format is Barracuda.
2014-07-22 14:59:47 9516 [Note] InnoDB: 128 rollback segment(s) are active.
2014-07-22 14:59:47 9516 [Note] InnoDB: Waiting for purge to start
2014-07-22 14:59:47 9516 [Note] InnoDB: 5.6.19 started; log sequence number 1626087
2014-07-22 14:59:47 9516 [Note] /usr/sbin/mysqld: ready for connections.
Version: '5.6.19' socket: '/var/lib/mysql/mysql.sock' port: 0 MySQL Community Server (GPL)
[root@DB-Server init.d]# mysql -u root mysql
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 1
Server version: 5.6.19 MySQL Community Server (GPL)
Copyright (c) 2000, 2014, 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 Host, User, Password,password_expired
-> from user
-> where user='root' and host='root' or host='localhost';
+-----------+------+-------------------------------------------+------------------+
| Host | User | Password | password_expired |
+-----------+------+-------------------------------------------+------------------+
| localhost | root | *A848DE7CCD839E924921BEE41711991DDA0D529E | Y |
+-----------+------+-------------------------------------------+------------------+
1 row in set (0.00 sec)
mysql> update user set password=PASSWORD('p12#456')
-> where user='root' and host='root' or host='localhost';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
新设置用户或更改密码后需用flush privileges刷新MySQL的系统权限相关表，否则会出现拒绝访问，还有一种方法，就是重新启动mysql服务器，来使新设置生效。
mysql> flush privileges;
Query OK, 0 rows affected (0.00 sec)
mysql> quit
Bye
[root@DB-Server init.d]# /etc/rc.d/init.d/mysql restart
Shutting down MySQL..140722 15:02:27 mysqld_safe mysqld from pid file /var/lib/mysql/DB-Server.pid ended
[ OK ]
Starting MySQL.[ OK ]
[2]- Done mysqld_safe --user=mysql --skip-grant-tables --skip-networking
[root@DB-Server init.d]# mysql -u root p
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)
[root@DB-Server init.d]# mysql -u root -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.6.19
Copyright (c) 2000, 2014, 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>
另外，如果登录mysql数据库后执行脚本遭遇 ERROR 1820 (HY000): You must SET PASSWORD before executing this statement,可以使用重新设置一次密码即可解决问题.
mysql>set password = password('p12#456');
至此问题解决，但是还有不少地方有些疑惑，为啥出现这个错误？ 其实这么多资料都只是说了解决方法，但是都回避了问题的原因。
参考资料：
http://huangyifa163.blog.163.com/blog/static/262875752011127102215790/
http://sundful.iteye.com/blog/704337
http://wenku.baidu.com/view/735ffa41be1e650e52ea995a.html
http://www.cnblogs.com/sunson/articles/2172086.html
http://blog.163.com/eric1945@126/blog/static/164934572201081494343373/
http://www.cnblogs.com/likai198981/archive/2013/04/06/3002518.html