Cek this syntax:

root@ubuntu:/opt/lampp# ./lampp startmysql
XAMPP: Starting MySQL...ok.
root@ubuntu:/opt/lampp# ./lampp status
Version: XAMPP for Linux 1.8.3-3
Apache is running.
MySQL is not running.
ProFTPD is running.

I already start mysql yet it's still unable to run. Why is it?

You need to start the mysqld service: sudo service mysqld start

root@ubuntu:/opt/lampp# sudo service mysqld start
mysqld: unrecognized service

----------------------------

root@ubuntu:/opt/lampp# ./lampp startmysql
XAMPP: Starting MySQL...ok.
root@ubuntu:/opt/lampp# ./lampp status
Version: XAMPP for Linux 1.8.3-3
Apache is running.
MySQL is not running.
ProFTPD is running.

As a sanity check:

dpkg -l | grep mysql

What is the output?

@davy
This means that the mysql server package has not been installed on your system. The mysql package is just the client. The mysqld service is installed as part of the mysql-server package.

You indicate that lamp started mysql ok. That may be a red herring. Look in /etc/init.d for anything that looks like mysql...

ls /etc/init.d

acpid              lightdm                      rfkill-store
acpi-support       modemmanager                 rsync
alsa-restore       module-init-tools            rsyslog
alsa-store         mysql                        

First stop the curepted running mysql. .

sudo service mysql stop

You have reconfigure the mysql installation. .

sudo dpkgreconfigure mysql

After that you have to set the password for mysql. .
and start mysql. .

sudo service mysql start

For MySQL 5.1 and such, the command "service mysqld start" is proper. For 5.6 (and possibly 5.5) it was changed to "service mysql start" - note the difference between mysqld and mysql in the commands.