CentOS安装mysql之后，之前设置了密码，但输入mysql -u -p后，会直接进入mysql，而输入mysql -uroot -p，则需要输入密码，这是为什么呢？
[root@hadoop01 ~]# mysql -u -p
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 11
Server version: 5.1.73 Source distribution
Copyright (c) 2000, 2013, 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> quit
Bye
[root@hadoop01 ~]# mysql -uroot -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 12
Server version: 5.1.73 Source distribution
Copyright (c) 2000, 2013, 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.
通过查找资料，原来是因为数据库里面有空用户。
解决方法如下：
进入mysql输入：
select * from mysql.user where user=”;
mysql> select * from mysql.user where user=”;
查询有结果，然后进行下一步。
use mysql;
delete from user where user = ”;
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> delete from user where user = '';
Query OK, 2 rows affected (0.00 sec)
删除了多余的空白账户， 然后进行下一步。
flush privileges;
mysql> flush privileges;
Query OK, 0 rows affected (0.00 sec)
重载一次权限表,最后用
service mysqld restart
[root@hadoop01 ~]# service mysqld restart
Stopping mysqld: [ OK ]
Starting mysqld: [ OK ]
重启mysql服务，问题得到解决，
[root@hadoop01 ~]# mysql -u -p
ERROR 1045 (28000): Access denied for user '-p'@'localhost' (using password: NO)
[root@hadoop01 ~]# mysql -uroot -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 3
Server version: 5.1.73 Source distribution
Copyright (c) 2000, 2013, 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> quit;
Bye
注意：
1、一定要记住重启mysql服务，否则不会生效。
2、msyql的用户表在mysql数据库中的user表中，主要字段有host，user，password等，作为mysql用的管理的主要表。