how do we know the root password of mysql code example

Example 1: mysql set root password

use mysql;

update user set authentication_string=PASSWORD("mynewpassword") where User='root';

flush privileges;

quit

Example 2: change mysql root password

$ sudo cat /etc/mysql/debian.cnf
Note the lines which read:

user     = debian-sys-maint
password = blahblahblah
Then:

$ mysql -u debian-sys-maint -p
Enter password: // type 'blahblahblah', ie. password from debian.cnf

mysql> USE mysql
mysql> SELECT User, Host, plugin FROM mysql.user;
+------------------+-----------+-----------------------+
| User             | Host      | plugin                |
+------------------+-----------+-----------------------+
| root             | localhost | auth_socket           |
| mysql.session    | localhost | mysql_native_password |
| mysql.sys        | localhost | mysql_native_password |
| debian-sys-maint | localhost | mysql_native_password |
+------------------+-----------+-----------------------+
4 rows in set (0.00 sec)

mysql> UPDATE user SET plugin='mysql_native_password' WHERE User='root';
mysql> COMMIT;  // When you don't have auto-commit switched on
Either:

mysql> ALTER USER 'root'@'localhost' IDENTIFIED BY 'new_password';
Or:

// For MySQL 5.7+
UPDATE mysql.user SET authentication_string=PASSWORD('new_password') where user='root';
Then:

mysql> FLUSH PRIVILEGES;
mysql> COMMIT;  // When you don't have auto-commit switched on
mysql> EXIT

$ sudo service mysql restart
$ mysql -u root -p
Enter password: // Yay! 'new_password' now works!