mysql-server : Depends: mysql-server-5.7 but it is not going to be installed

The error:

mysql-server : Depends: mysql-server-5.7 but it is not going to be installed

happens when there is either

1. MariaDB installed and then removed
2. Something went wrong with the upgrade and downgrade

To fix,

/var/lib/mysql/debian-*.flag and try installing again.

rm -f /var/lib/mysql/debian-10.0.flag

Try apt -f install

and apt install mysql-server