How to get the difference between two dates rounded to hours

I think a more complete answer is

select time_format(timediff(onedateinstring,anotherdateinstring),'%H:%i:%s')

This allows you to see the hours, minutes, seconds, etc. that you wish to see in the format you want...

More information on time_format: http://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_time-format

select time_to_sec(timediff('2010-09-01 03:00:00', '2010-09-01 00:10:00' )) / 3600;

+-----------------------------------------------------------------------------+
| time_to_sec(timediff('2010-09-01 03:00:00', '2010-09-01 00:10:00' )) / 3600 |
+-----------------------------------------------------------------------------+
|                                                                      2.8333 | 
+-----------------------------------------------------------------------------+

Use TIMESTAMPDIFF for exact number of hours :

SELECT 
      b.`Start_Date`, 
      b.`End_Date`, 
      TIMESTAMPDIFF(HOUR, b.`Start_Date`, b.`End_Date`) AS `HOURS` 
FROM my_table b;