This appears to be a common problem on 12.04. I\'ve tried everthing in this thread including:
sudo apt-get clean
sudo apt-get autoclean
sudo apt-get remove
I tried everything above, but it still wouldn't install. I finally got it working with this post: http://www.randomhacks.co.uk/mysql-job-failed-to-start-unable-to-set-root-password-cant-find-file-mysql/
It advises to remove folders similar to what was stated above, and also to run
dpkg --get-selections | grep mysql
sudo apt-get remove --purge [everything that came up]
sudo apt-get autoremove && sudo apt-get autoclean
sudo apt-get install mysql-server