The error:
mysql-server : Depends: mysql-server-5.7 but it is not going to be installed
happens when there is either
- MariaDB installed and then removed
- 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