2012-06-01

warning: database file for 'extra' does not exist
warning: database file for 'community' does not exist

error: failed to prepare transaction (could not find database)

Firstly, try running pacman -Syy, then try to install sudo again.

Check that the repositories are uncommented in /etc/pacman.conf.

Or your mirrorlist might be outdated: Generate a current list of mirrors and copy it to /etc/pacman.d/mirrorlist

Quoting from this relevant forum thread:

You can:
pick another mirror
try using an http mirror, not an ftp one (pick http mirror from the mirrorlist).


Alternatively you can manually download the databases with:
wget ftp://mirror.csclub.uwaterloo.ca/archlinux/community/os/x86_64/community.db
wget ftp://mirror.csclub.uwaterloo.ca/archlinux/extra/os/x86_64/extra.db

move them to /var/lib/pacman/sync/ and run 'pacman -Syu' again. If you find any *.part files in /var/lib/pacman/sync/ e.g. /var/lib/pacman/sync/core.db.part - remove them.

To prevent having problems like these it is critical to understand pacman. To learn more about using pacman, see the ArchWiki pacman article, and consult man pacman.