Database error on Debian 9

While running on `Debian GNU/Linux 9 (stretch)` everything goes fine but when you visit the site, it shows **Error establishing a database connection** 
Even trying to log in to SQL shell with given password shows error. 
```sh
$ mysql -u root -p`
ERROR 1698 (28000): Access denied for user 'root'@'localhost'
```
Seems like it is related to the way we setup `mysql-server`

