PHP Mysql从一天中的两个日期时间计算每天的总飞行小时数

Row_id Sensor_ID  datetime_takeoff       datetime_landing
1      SFO        2013-09-18 04:34:22    2013-09-19 08:34:22
2      BWI        2013-09-18 04:34:22    2013-09-18 16:55:23 
3      BWI        2013-09-18 20:34:22    2013-09-19 10:34:22   
4      SFO        2013-09-19 15:21:22    2013-09-19 20:34:22   
5      BWI        2013-09-19 20:34:22    2013-09-20 06:15:16   
6      SFO        2013-09-19 23:47:22    2013-09-20 07:59:59   
7      BWI        2013-09-20 11:34:05    2013-09-21 02:05:21   
8      SFO        2013-09-20 10:28:56    NULL   
9      BWI        2013-09-21 04:09:57    NULL         
Row_id Sensor_ID  datetime_takeoff       dateti