On APT-based systems

1. Check whether there are any existing packages:

shell> dpkg -l | grep -i mysql

1. Remove the packages using the following:

shell> sudo apt-get remove mysql-community-server mysql-client mysql-common mysql-community-client mysql-community-client-core mysql-community-server mysql-community-server-core -y
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages will be REMOVED:
  mysql-client mysql-common mysql-community-client mysql-community-client-core mysql-community-server mysql-community-server-core mysql-server
0 upgraded, 0 newly installed, 7 to remove and 341 not upgraded.
After this operation, 357 MB disk space will be freed.
(Reading database ... 134358 files and directories currently installed.)
Removing mysql-server (8.0.3-rc-1ubuntu16.04) ...
Removing mysql-community-server (8.0.3-rc-1ubuntu16.04) ...
update-alternatives: using /etc/mysql/my.cnf.fallback to provide /etc/mysql/my.cnf (my.cnf) in auto mode
Removing mysql-client (8.0.3-rc-1ubuntu16.04) ...
Removing mysql-community-client (8.0.3-rc-1ubuntu16.04) ...
Removing mysql-common (8.0.3-rc-1ubuntu16.04) ...
Removing mysql-community-client-core (8.0.3-rc-1ubuntu16.04) ...
Removing mysql-community-server-core (8.0.3-rc-1ubuntu16.04) ...
Processing triggers for man-db (2.7.5-1) ...

Or remove them using:

shell> sudo apt-get remove --purge mysql-\* -y
shell> sudo apt-get autoremove -y

1. Verify that the packages are uninstalled:

shell> dpkg -l | grep -i mysql
ii  mysql-apt-config        0.8.9-1               all          Auto configuration for MySQL APT Repo.
rc  mysql-common            8.0.3-rc-1ubuntu16.04 amd64        MySQL Common
rc  mysql-community-client  8.0.3-rc-1ubuntu16.04 amd64        MySQL Client
rc  mysql-community-server  8.0.3-rc-1ubuntu16.04 amd64        MySQL Server

rc indicates that the packages have been removed (r), and only config files (c) have been kept.