SELECT datediff( hour, '4:00AM', '5:50AM' ) --返回 1
SELECT datediff( month, '1987/05/02', '1995/11/15' ) --返回 102
SELECT datediff( day, '00:00', '23:59' ) --返回 0
SELECT datediff( day,'1999/07/19 00:00','1999/07/23 23:59' ) --返回 4
SELECT datediff( month, '1999/07/19', '1999/07/23' ) --返回 0
SELECT datediff( month, '1999/07/19', '1999/08/23' ) --返回 1