:~$ sudo apt-get install mariadb-server
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:
 mariadb-server : Depends: mariadb-server-10.0 (>= 10.0.29-0ubuntu0.16.04.1) but it is not going to be installed
 mysql-server : Depends: mysql-server-5.7 but it is not going to be installed
E: Unmet dependencies. Try 'apt-get -f install' with no packages (or specify a solution).

$ sudo apt-get -f install
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Correcting dependencies... Done
The following packages were automatically installed and are no longer required:
  libdbd-mysql-perl libdbi-perl libterm-readkey-perl mariadb-common
Use 'sudo apt autoremove' to remove them.
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.
7 not fully installed or removed.
Need to get 0 B/2,554 kB of archives.
After this operation, 48.3 MB of additional disk space will be used.
Do you want to continue? [Y/n] 
Preconfiguring packages ...
(Reading database ... 118290 files and directories currently installed.)
Preparing to unpack .../mysql-server-5.7_5.7.18-0ubuntu0.16.04.1_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_5.7.18-0ubuntu0.16.04.1_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_5.7.18-0ubuntu0.16.04.1_amd64.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)