众所周知，DATEDIFF() 函数用于获取两个日期之间的天数差。因此，它也很可能返回负值。
mysql> select * from differ; +------------+-------------+ | OrderDate | WorkingDate | +------------+-------------+ | 2017-10-22 | 2017-10-29 | | 2017-10-25 | 2017-10-30 | | 2017-10-25 | 2017-11-30 | +------------+-------------+ 3 rows in set (0.00 sec)
