title: ifnull not working on mysql
body: <p>I have the following table:</p>
  
  <pre>
  +----+------------+----------+------------------+
  | id | created_at | platform | platform_version |
  +----+------------+----------+------------------+
  |  1 |          1 | mac      | 1                |
  |  2 |          2 | mac      | 1                |
  |  3 |          3 | mac      | 2                |
  |  4 |          4 | mac      | 2                |
  |  5 |          5 | mac      | 2                |
  |  6 |          5 | mac      | 3                |
  |  7 |          2 | windows  | 1                |
  |  8 |          2 | windows  | 2                |
  |  9 |          3 | windows  | 3                |
  | 10 |          3 | windows  | 1                |
  | 11 |          4 | windows  | 2                |
  | 12 |          4 | windows  | 3                |
  | 13 |          5 | windows  | 4                |
  | 14 |          5 | windows  | 1                |
  | 15 |          6 | windows  | 2                |
  | 16 |          6 | windows  | 3                |
  +----+------------+----------+------------------+
  </pre>
  
  <p>I want to have a result like the following:</p>
  
  <pre>
  +-------------+---------------+---------------+------------+
  | group_count | running_total | windows_total | created_at |
  +-------------+---------------+---------------+------------+
  |           1 |             1 |             0 |          1 |
  |           6 |             7 |             4 |          2 |
  |           7 |            14 |             8 |          4 |
  |           2 |            16 |            10 |          6 |
  +-------------+---------------+---------------+------------+
  </pre>
  
  <p>But when I execute this select statement:</p>
  
  <pre>
  SELECT group_count,
         (@r := @r + group_count ) AS running_total,
         (@w := @w + ifnull( win_count, 0 )) AS windows_total,
         t1.created_at
  FROM  (SELECT   (@r :=0),
                  COUNT( * ) AS group_count,
                  platform,
                  created_at
         FROM     devices
         GROUP BY created_at DIV 2
        ) AS t1 LEFT JOIN
        (SELECT   COUNT( * ) AS win_count,
                  created_at
         FROM     devices
         WHERE    platform = 'windows'
         GROUP BY created_at DIV 2
        ) AS t3 ON t1.created_at = t3.created_at
  </pre>
  
  <p>It comes out like this:</p>
  
  <pre>
  +-------------+---------------+---------------+------------+
  | group_count | running_total | windows_total | created_at |
  +-------------+---------------+---------------+------------+
  |           1 |             1 |          NULL |          1 |
  |           6 |             7 |          NULL |          2 |
  |           7 |            14 |          NULL |          4 |
  |           2 |            16 |          NULL |          6 |
  +-------------+---------------+---------------+------------+
  </pre>
  
  <p>coalesce also doesn't work. Could anyone help? Thanks.</p>