apt-get purge mysql-server
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages will be REMOVED:
  mysql-server*
0 upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
2 not fully installed or removed.
After this operation, 184 kB disk space will be freed.
Do you want to continue? [Y/n] Y
(Reading database ... 385757 files and directories currently installed.)
Removing mysql-server (5.7.11-0ubuntu6) ...
Setting up mysql-server-5.7 (5.7.11-0ubuntu6) ...
Job for mysql.service failed because the control process exited with error code. See "systemctl status mysql.service" and "journalctl -xe" for details.
invoke-rc.d: initscript mysql, action "start" failed.
dpkg: error processing package mysql-server-5.7 (--configure):
 subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
 mysql-server-5.7
E: Sub-process /usr/bin/dpkg returned an error code (1)