When failed to start MariaDB database server (Solution - Again Installation)

In the tutorial you should install mariadb-server in place of mysql-server.

sudo apt purge mysql*
sudo apt purge mariadb*
sudo apt autoremove
sudo apt autoclean

Check if still something in the system :

dpkg -l | grep -e mysql -e mariadb 
if the list is empty then install mariadb server :

sudo apt install mariadb-server
Now try to check status on stop and start the database :

Status after install :

sudo systemctl status mysql.service
● mariadb.service - MariaDB database server
   Loaded: loaded (/lib/systemd/system/mariadb.service; enabled; vendor preset: enabled)
   Active: active (running) since Wed 2018-03-21 16:08:49 CET; 2min 1s ago
 Main PID: 15699 (mysqld)
   Status: "Taking your SQL requests now..."
   CGroup: /system.slice/mariadb.service
           └─15699 /usr/sbin/mysqld

.../..
Check the PID :

sudo more /var/run/mysqld/mysqld.pid 
15699
Stop the database :

sudo systemctl stop mysql.service
sudo systemctl status mysql.service
● mariadb.service - MariaDB database server
   Loaded: loaded (/lib/systemd/system/mariadb.service; enabled; vendor preset: enabled)
   Active: inactive (dead) since Wed 2018-03-21 16:14:35 CET; 1s ago
 Main PID: 15699 (code=exited, status=0/SUCCESS)
   Status: "MariaDB server is down"

.../...
You could check the PID file is not present as well

sudo more /var/run/mysqld/mysqld.pid 
more: stat on /var/run/mysqld/mysqld.pid: No file or folder
Start the database :

sudo systemctl start mysql.service
sudo systemctl status mysql.service
● mariadb.service - MariaDB database server
   Loaded: loaded (/lib/systemd/system/mariadb.service; enabled; vendor preset: enabled)
   Active: active (running) since Wed 2018-03-21 16:14:54 CET; 3s ago
  Process: 15911 ExecStartPost=/bin/sh -c systemctl unset-environment _WSREP_START_POSITION (code=exited, status=0/SUCCESS)
  Process: 15908 ExecStartPost=/etc/mysql/debian-start (code=exited, status=0/SUCCESS)
  Process: 15791 ExecStartPre=/bin/sh -c [ ! -e /usr/bin/galera_recovery ] && VAR= ||   VAR=`/usr/bin/galera_recovery`; [ $? -eq 0 ]   && systemctl set-environment _WSREP_S
  Process: 15787 ExecStartPre=/bin/sh -c systemctl unset-environment _WSREP_START_POSITION (code=exited, status=0/SUCCESS)
  Process: 15784 ExecStartPre=/usr/bin/install -m 755 -o mysql -g root -d /var/run/mysqld (code=exited, status=0/SUCCESS)
 Main PID: 15881 (mysqld)
   Status: "Taking your SQL requests now..."
    Tasks: 26 (limit: 4915)
   CGroup: /system.slice/mariadb.service
           └─15881 /usr/sbin/mysqld
You could check the PID file it should be present. May help on a empty database. If there is something in the database you should do a backup first.