You have forgotten to execute your statement. After you prepare, you must $sth->execute your statement before $sth->fetching results. As you haven't executed your statement, it contains no results.
Alternatively, try fiddling with $dbh->selectall_arrayref or $dbh->selectall_hashref, which does the preparing, executing and fetching for you in one go.
Comment on Re: Use DBI to select and return multiple database entries