Cannot install mysql-server-5.5/the following packages have unmet dependicies

Open the terminal and type:

sudo apt-get purge mysql-client-core-5.6
sudo apt-get autoremove
sudo apt-get autoclean
sudo apt install mysql-client-core-5.5
sudo apt install mysql-server  

Both MySQL database core client and MySQL Server packages will be the same version 5.5. MySQL Client 5.5 and MySQL Server 5.5 are the current "best" versions of these packages in Ubuntu 14.04 as determined by the package maintainers. If you would rather install MySQL Client 5.6 and MySQL Server 5.6 you can also find the mysql-client-core-5.6 and mysql-server-5.6 packages in the Ubuntu Software Center. The important thing is that the client and server version numbers match in either case.

In Ubuntu 16.04 and 18.04 the names of these two packages are mysql-client-core-5.7 and mysql-server-5.7.

Following command has solved my problem.

sudo aptitude install mysql-server

If aptitude is not installed in your computer, first install aptitude using following command.

sudo apt-get install aptitude