title: Sort comments by upvotes-downvotes, make sure ones without votes are displayed above negative
body: <p>I'm fetching comments and sorting them by their score (<code>upvotes-downvotes</code>). However if a comment has not been voted on at all, then it has NULL upvotes &amp; downvotes, and so ones that are negative will be displayed first.</p>
  
  <p>Upvotes are <code>cv_type = 1</code>, downvotes are <code>cv_type = 0</code>.</p>
  
  <pre><code>SELECT c_id,c_text,c_date,c_parentid,u_username,
  (select sum(IF( cv.cv_type=  '1', 1 ,  0 )) from comment_votes cv where cv_commentid=comments.c_id ) as upvotes,
  (select sum(IF( cv.cv_type=  '0', 1 ,  0 )) from comment_votes cv where cv_commentid=comments.c_id ) as downvotes
  FROM comments
  INNER JOIN users
  ON u_id = c_userid
  WHERE c_postid = ? AND c_parentid = 0
  ORDER BY upvotes-downvotes
  DESC LIMIT 100
  </code></pre>
  
  <p>Any way to make sure ones without votes are above ones that are negative?</p>