Access denied for user ‘root@localhost’ (using password:NO)

for this kind of error; you just have to set new password to the root user as an admin. follow the steps as follows:

[root ~]# mysql -u root
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password:NO)

1. Stop the service/daemon of mysql running

   [root ~]# service mysql stop   
   mysql stop/waiting
   

2. Start mysql without any privileges using the following option;
   This option is used to boot up and do not use the privilege system of MySQL.

   [root ~]# mysqld_safe --skip-grant-tables &
   

At this moment, the terminal will seem to halt. Let that be, and use new terminal for next steps.

3. enter the mysql command prompt

   [root ~]# mysql -u root
   mysql> 
   

4. Fix the permission setting of the root user ;

   mysql> use mysql;
   Database changed
   mysql> select * from  user;
   Empty set (0.00 sec)
   mysql> truncate table user;
   Query OK, 0 rows affected (0.00 sec)
   mysql> flush privileges;
   Query OK, 0 rows affected (0.01 sec)
   mysql> grant all privileges on *.* to root@localhost identified by 'YourNewPassword' with grant option;
   Query OK, 0 rows affected (0.01 sec)
   

*if you don`t want any password or rather an empty password

    mysql> grant all privileges on *.* to root@localhost identified by '' with grant option;
    Query OK, 0 rows affected (0.01 sec)*
    mysql> flush privileges;
    Query OK, 0 rows affected (0.00 sec)

Confirm the results:

    mysql> select host, user from user;
+-----------+------+
| host      | user |
+-----------+------+
| localhost | root |
+-----------+------+
1 row in set (0.00 sec)

5. Exit the shell and restart mysql in normal mode.

   mysql> quit;
   [root ~]# kill -KILL [PID of mysqld_safe]
   [root ~]# kill -KILL [PID of mysqld]
   [root ~]# service mysql start
   

6. Now you can successfully login as root user with the password you set

    [root ~]# mysql -u root -pYourNewPassword 
    mysql>