CREATE TABLE test_timediff
( id INTEGER NOT NULL PRIMARY KEY AUTO_INCREMENT
, start_datetime DATETIME
, end_datetime DATETIME
);
INSERT INTO test_timediff VALUES
 ( 1, '2009-02-13 23:00:00','2009-02-13 00:30:00' )
,( 2, '2009-02-14 23:00:00','2009-02-13 00:30:00' )
,( 3, '2009-02-13 23:00:00','2009-02-14 00:30:00' )
;
SELECT t.*
     , TIMEDIFF(start_datetime,end_datetime) AS diff1
     , TIMEDIFF(end_datetime,start_datetime) AS diff2
  FROM test_timediff AS t
;
/* results:
id start_datetime      end_datetime          diff1     diff2
 1 2009-02-13 23:00:00 2009-02-13 00:30:00  22:30:00 -22:30:00
 2 2009-02-14 23:00:00 2009-02-13 00:30:00  46:30:00 -46:30:00
 3 2009-02-13 23:00:00 2009-02-14 00:30:00 -01:30:00  01:30:00
*/