mysql timediff to hours

TIMEDIFF(endDate, startDate) outputs in DateTime format, so flat that to timestamp and devide by (60*60)

SELECT (UNIX_TIMESTAMP(TIMEDIFF(endDate, startDate))/(60*60)) AS hours_difference
FROM tasks

Edit: Alternatively,TimestampDiff may also provide a valid solution in more elegant way providing its example:

SELECT TIMESTAMPDIFF(MONTH,'2003-02-01','2003-05-01');

And your solution can be:

SELECT TIMESTAMPDIFF(HOUR, startDate, endDate) AS hours_different
FROM tasks