Updated mapper and query to support build-up of select statement and passing a null to reset the fields array. You can now do something like

$mapper->all('Entity')
    ->select(array('id', 'name'))
    ->select('category_long_name AS cname')
    ->select("CASE WHEN (date_created > '2012-12-01') THEN 'y' ELSE 'n' END AS past_date");