SELECT datediff( pr_endtime, pr_starttime ) AS date_diff FROM problem_reports;

SELECT TIME_TO_SEC(TIMEDIFF(pr_endtime,pr_starttime))/86400 FROM problem_reports;

SELECT TIME_TO_SEC( TIMEDIFF( pr_endtime, pr_starttime))/3600 FROM problem_reports;



http://ellislab.com/forums/viewthread/186132/P190
http://www.grocerycrud.com/documentation/options_functions/callback_column


DROP TRIGGER [IF EXISTS] [schema_name.]trigger_name;

DROP TRIGGER IF EXISTS diff_datetime;
CREATE
        TRIGGER `diff_datetime` BEFORE UPDATE
        ON `problem_reports` FOR EACH ROW
                set new.pr_duration = TIME_TO_SEC( TIMEDIFF( new.pr_endtime, new.pr_starttime))/60;
delimiter ;





ALTER TABLE `problem_reports` ADD `pr_duration` VARCHAR( 30 ) NOT NULL