SUBDATE(date,INTERVAL expr unit) 和 SUBDATE(expr,days)
当第二个参数INTERVAL形式调用， SUBDATE() 是DATE_SUB()的同义词，有关INTERVAL单位参数信息，请参阅DATE_ADD()的讨论。
mysql> SELECT DATE_SUB('1998-01-02', INTERVAL 31 DAY);
+---------------------------------------------------------+
| DATE_SUB('1998-01-02', INTERVAL 31 DAY) |
+---------------------------------------------------------+
| 1997-12-02 |
+---------------------------------------------------------+
1 row in set (0.00 sec)
mysql> SELECT SUBDATE('1998-01-02', INTERVAL 31 DAY);
+---------------------------------------------------------+
| SUBDATE('1998-01-02', INTERVAL 31 DAY) |
+---------------------------------------------------------+
| 1997-12-02 |
+---------------------------------------------------------+
1 row in set (0.00 sec)
