~ $ sudo aptitude -y install mysql-server
The following NEW packages will be installed:
  mysql-server mysql-server-5.7{a} mysql-server-core-5.7{a} 
0 packages upgraded, 3 newly installed, 0 to remove and 1 not upgraded.
Need to get 0 B/10.0 MB of archives. After unpacking 94.5 MB will be used.
Preconfiguring packages ...              
Selecting previously unselected package mysql-server-core-5.7.
(Reading database ... 320213 files and directories currently installed.)
Preparing to unpack .../mysql-server-core-5.7_5.7.17-0ubuntu0.16.04.2_amd64.deb ...
Unpacking mysql-server-core-5.7 (5.7.17-0ubuntu0.16.04.2) ...
Preparing to unpack .../mysql-server-5.7_5.7.17-0ubuntu0.16.04.2_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_5.7.17-0ubuntu0.16.04.2_amd64.deb (--unpack):
 subprocess new pre-installation script returned error exit status 1
Selecting previously unselected package mysql-server.
Preparing to unpack .../mysql-server_5.7.17-0ubuntu0.16.04.2_all.deb ...
Unpacking mysql-server (5.7.17-0ubuntu0.16.04.2) ...
Processing triggers for man-db (2.7.5-1) ...
Errors were encountered while processing:
 /var/cache/apt/archives/mysql-server-5.7_5.7.17-0ubuntu0.16.04.2_amd64.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)
Failed to perform requested operation on package.  Trying to recover:
dpkg: dependency problems prevent configuration of mysql-server:
 mysql-server depends on mysql-server-5.7; however:
  Package mysql-server-5.7 is not installed.

dpkg: error processing package mysql-server (--configure):
 dependency problems - leaving unconfigured
Setting up mysql-server-core-5.7 (5.7.17-0ubuntu0.16.04.2) ...
Errors were encountered while processing:
 mysql-server
                                         
Current status: 1 (+1) broken.