Time calculations with MySQL TIMEDIFF

I have the following table:

mysql> SELECT id,start1,stop1,start2,stop2 FROM times;
+----+---------------------+---------------------+---------------------+---------------------+
| id | start1              | stop1               | start2              | stop2               |
+----+---------------------+---------------------+---------------------+---------------------+
|  4 | 2010-04-23 08:05:00 | 2010-04-23 12:15:00 | 2010-04-23 12:45:00 | 2010-04-23 16:50:00 | 
|  2 | 2010-04-26 09:30:00 | 2010-04-26 12:10:00 | 2010-04-26 12:50:00 | 2010-04-26 16:50:00 | 
|  7 | 2010-04-28 08:45:00 | 2010-04-28 11:45:00 | 2010-04-28 13:10:00 | 2010-04-28 17:29:00 | 
|  6 | 2010-04-27 09:30:00 | 2010-04-27 12:15:00 | 2010-04-27 12:55:00 | 2010-04-27 18:44:00 | 
+----+---------------------+---------------------+---------------------+---------------------+

I want to sum total worktime and the difference to the "needed work hours". It works pretty well with the statement below, but for unknown reasons it doesn't work for id 6. start*/stop* fields are in format datetime.

SELECT *, TIME_FORMAT(TIMEDIFF(totaltime,'08:24'),'%H:%i') AS diff,
totaltime > '08:24' AS redorgreen FROM
(
SELECT 
    DATE_FORMAT(start1,'%a %e. %M %Y') AS date,
    T开发者_如何学PythonIME_FORMAT(SUM(TIMEDIFF(stop1,start1) + TIMEDIFF(stop2,start2)),'%H:%i') AS totaltime,
    TIME_FORMAT(start1,'%H:%i') AS start1,
    TIME_FORMAT(stop1,'%H:%i') AS stop1,
    TIME_FORMAT(start2,'%H:%i') AS start2,
    TIME_FORMAT(stop2,'%H:%i') AS stop2,
    id as id
FROM times GROUP BY id ASC
) AS somethingwedontneed;

This is the result:

select id,
       TIME_FORMAT(SUM(TIMEDIFF(stop1,start1) + TIMEDIFF(stop2,start2)),'%H:%i') 
       AS totaltime from times group by id;
+----+-----------+
| id | totaltime |
+----+-----------+
|  2 | 06:40     | 
|  4 | 08:15     | 
|  6 | NULL      | 
|  7 | 07:19     | 
+----+-----------+

Thanks in advance for every hint.

SELECT id, TIMEDIFF( stop1, start1 ) , TIMEDIFF( stop2, start2 ) , ADDTIME( TIMEDIFF( stop1, start1 ) , TIMEDIFF( stop2, start2 ) ) , TIME_FORMAT( ADDTIME( TIMEDIFF( stop1, start1 ) , TIMEDIFF( stop2, start2 ) ) , '%H:%i' ) AS totaltime FROM times GROUP BY id

Try to wrap '08:24' in TIME(), that is TIME('08:24').