title: MySQL group rows to find time difference
body: <p>I have <code>car_history</code> table, I need to find time difference between each <code>car_status</code> in selected date.</p>
  
  <p>e.g  <code>1. to_load - 9 h. 2. to_load - 8 h.</code> and so on...</p>
  
  <p>How I can find 
  entries from <code>id = 1 to id = 4</code> and <code>id = 8 to id = 10</code> to count how log took <code>to_load</code> status. </p>
  
  <pre><code>  --------------------------------------------------------------------
    |  id | car_id | car_status | comments        | entry_date         |
    --------------------------------------------------------------------
    |  1  |   1    | to_load    | preparing       | 2013-06-12 08:00:00|
    |  2  |   1    | to_load    | preparing goods | 2013-06-12 14:10:00|
    |  3  |   1    | to_load    | loading goods   | 2013-06-12 17:00:50|
    |  4  |   1    | loaded     | finding driver  | 2013-06-13 07:00:00|
    |  5  |   1    | loaded     | ready to deliver| 2013-06-13 08:00:00|
    |  6  |   1    | on_road    | delivering goods| 2013-06-13 09:00:00|
    |  7  |   1    | in_garage  | goods delivered | 2013-06-13 15:00:00|
    |  8  |   1    | to_load    | preparing       | 2013-06-14 08:00:00|
    |  9  |   1    | to_load    | preparing goods | 2013-06-14 14:10:00|
    |  10 |   1    | to_load    | loading goods   | 2013-06-14 16:00:50|
    |  11 |   1    | loaded     | finding driver  | 2013-06-12 08:00:00|
    --------------------------------------------------------------------
  </code></pre>