First add the MySQL repo the the sources.list file:

echo -e "deb stretch mysql-5.7\ndeb-src stretch mysql-5.7" > /etc/apt/sources.list.d/mysql.list 

Then we are going to download and import the GPG key:

wget -O /tmp/RPM-GPG-KEY-mysql
 apt-key add /tmp/RPM-GPG-KEY-mysql

And then we can install the mysql-server:

apt update
apt install mysql-server

Note: Make sure to run apt update before installing MySQL. If you don’t to that the default MariaDB server will be installed.