sudo apt update
sudo apt install mysql-server
sudo service mysql start
sudo mysql_secure_installation
sudo mysql -u root 

In mysql console:
DROP USER 'root'@'localhost';
CREATE USER 'root'@'%' IDENTIFIED BY 'YOURPASSWORD';
GRANT ALL PRIVILEGES ON *.* TO 'root'@'%' WITH GRANT OPTION;
FLUSH PRIVILEGES;

sudo /etc/init.d/mysql start
or 
systemctl start mysql.service

sudo apt install mysql-server
sudo mysql_secure_installation utility

sudo apt-get update
sudo apt-get install mysql-server

$ mysql
$ sudo apt-get install mysql-server
$ mysql -u root -p
$ sudo mysql -u root -p
$ use mysql;
$ update user set plugin=”mysql_native_password” where user=’root’;
$ flush privileges;
$ exit
$ mysql -u root -p
$ create database demo;
$ show databases;

//follow this blog

https://www.digitalocean.com/community/tutorials/how-to-install-mysql-on-ubuntu-18-04