title: ROLLUP returning incorrect value
body: <p>Stuck on this for a bit today and was wondering can anyone suggest what's going wrong.
  Here is my code:</p>
  
  <pre><code>select g.game_name, r.rental_id, sum(if(datediff(r.return_date, r.due_date) &gt; 0, datediff(r.return_date, r.due_date) * g.overdue_charge_per_day, 0)) Overdue_Charges
  from rental as r
  left join game as g
  on r.game_id = g.game_id
  where month(r.return_date)=month(now())
  group by rental_id
  with rollup;
  </code></pre>
  
  <p>This displays the following:</p>
  
  <p>GTA V 12 4.00 </p>
  
  <p>Tony Hawk 13 15.00 </p>
  
  <p>Tony Hawk null 19.00</p>
  
  <p>So it totals 4 &amp; 15. However I don't want it to display "Tony Hawk" twice. Can I replace this with Grand Total? Maybe it's a joining issue and the second Tony Hawk should also be null.
  Thanks</p>