package:mysql-server-5.7:(not installed)
Preparing to unpack .../mysql-server-5.7_5.7.13-0ubuntu0.16.04.2_amd64.deb ...
Aborting downgrade from (at least) 10.0 to 5.7.
If are sure you want to downgrade to 5.7, remove the file
/var/lib/mysql/debian-*.flag and try installing again.
dpkg: error processing archive /var/cache/apt/archives/mysql-server-5.7_5.7.13-0ubuntu0.16.04.2_amd64.deb (--unpack):
 subprocess new pre-installation script returned error exit status 1