MySQL is a well known free, open-source relational database service, and it's great. MariaDB is just MySQL (a fork of it), but better.
Install MariaDB
sudo apt install mariadb-server
Secure Install/Setup
sudo mysql_secure_installation
Create Admin user
sudo mysql
GRANT ALL ON *.* TO 'admin'@'localhost' IDENTIFIED BY 'password' WITH GRANT OPTION;
FLUSH PRIVILEGES;
exit;
Test it works
(Optional) Make it easier to access on command line
If you're working with a terminal, when calling mysql you'll need to enter a password each time. You can store the passwordwith a special .cnf file, making it faster to get into writing SQL.
Create/Edit the file with your editor of choice (mine's vim)
vim ~/.my.cnf
Add the following, with your credentials
[mysql]
user=
password=
The above can be used for mysqldump, mysqladmin, and others too, by replacing the [mysql] block