root@vps-zap373328-2:~# apt-get install mysql-server-5.5
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
  libreadline5 mariadb-common
Use 'apt-get autoremove' to remove them.
Suggested packages:
  tinyca
The following NEW packages will be installed:
  mysql-server-5.5
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
6 not fully installed or removed.
Need to get 0 B/1878 kB of archives.
After this operation, 32.5 MB of additional disk space will be used.
Preconfiguring packages ...
(Reading database ... 23546 files and directories currently installed.)
Preparing to unpack .../mysql-server-5.5_5.5.62-0+deb8u1_amd64.deb ...
Aborting downgrade from (at least) 10.0 to 5.5.
If are sure you want to downgrade to 5.5, remove the file
/var/lib/mysql/debian-*.flag and try installing again.
dpkg: error processing archive /var/cache/apt/archives/mysql-server-5.5_5.5.62-0+deb8u1_amd64.deb (--unpack):
 subprocess new pre-installation script returned error exit status 1
Errors were encountered while processing:
 /var/cache/apt/archives/mysql-server-5.5_5.5.62-0+deb8u1_amd64.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)
root@vps-zap373328-2:~# apt-get  install apache2
Reading package lists... Done
Building dependency tree
Reading state information... Done
apache2 is already the newest version.
You might want to run 'apt-get -f install' to correct these:
The following packages have unmet dependencies:
 mysql-server : Depends: mysql-server-5.5 but it is not going to be installed
E: Unmet dependencies. Try 'apt-get -f install' with no packages (or specify a solution).