apt error after MariaDB to MySQL change

I have install MariaDB on a Ubuntu 16.04 server. Then installed Virtualmin, that require MySQL, so it removed MariaDB and installed MySQL, this caused apt stop working.

When i run “apt update”, it failed with

root@ok-vm:~# apt upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
You might want to run 'apt-get -f install' to correct these.
The following packages have unmet dependencies:
 mysql-server : Depends: mysql-server-5.7 but it is not installed
E: Unmet dependencies. Try using -f.
root@ok-vm:~#

Next i run ‘apt-get -f install’ as it said in above error message.

root@ok-vm:~# apt-get -f install
Reading package lists... Done
Building dependency tree
Reading state information... Done
Correcting dependencies... Done
The following additional packages will be installed:
  mysql-server-5.7
Suggested packages:
  mailx tinyca
The following NEW packages will be installed:
  mysql-server-5.7
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
167 not fully installed or removed.
Need to get 0 B/2,708 kB of archives.
After this operation, 48.3 MB of additional disk space will be used.
Do you want to continue? [Y/n] Y
Preconfiguring packages ...
(Reading database ... 120876 files and directories currently installed.)
Preparing to unpack .../mysql-server-5.7_5.7.20-0ubuntu0.16.04.1_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_5.7.20-0ubuntu0.16.04.1_amd64.deb (--unpack):
 subprocess new pre-installation script returned error exit status 1
Errors were encountered while processing:
 /var/cache/apt/archives/mysql-server-5.7_5.7.20-0ubuntu0.16.04.1_amd64.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)
root@ok-vm:~#

That too failed. The error message is

Preparing to unpack …/mysql-server-5.7_5.7.20-0ubuntu0.16.04.1_amd64.deb …
Aborting downgrade from (at least) 10.0 to 5.7.

This is because MySQL detected MairaDB data files in /var/lib/mysql folder, it can’t use same data files as MariaDB is newer version.

To fix the problem, i renamed the folder

mv /var/lib/mysql /var/lib/mysql-old

Now running “apt-get -f install” fixed the problem with apt.

apt-get -f install