timestampdiff
返回 datetime_expr2 和 datetime_expr1 的差值，其中 datetime_expr1 和datetime_expr2 是日期或日期时间表达式。
结果(整数)的单位由 unit 参数给出。interval 的单位由 unit 参数给出，应该是下列值之一:
MILLISECOND（3.2 及以后），SECOND，MINUTE，HOUR，DAY，WEEK，MONTH，YEAR。
语法
BIGINT TIMESTAMPDIFF(unit, DATETIME datetime_expr1, DATETIME datetime_expr2)
参数说明
datetime_expr: 要比较的日期或日期时间表达式。unit：时间差值的单位。支持的单位包括 MILLISECOND (3.2 及以后)，SECOND，MINUTE，HOUR，DAY，WEEK，MONTH，YEAR。
返回值说明
返回 BIGINT 类型的值。
示例
MySQL > SELECT TIMESTAMPDIFF(MONTH,'2003-02-01','2003-05-01');
+--------------------------------------------------------------------+
| timestampdiff(MONTH, '2003-02-01 00:00:00', '2003-05-01 00:00:00') |
+--------------------------------------------------------------------+
| 3 |
+--------------------------------------------------------------------+
MySQL > SELECT TIMESTAMPDIFF(YEAR,'2002-05-01','2001-01-01');
+-------------------------------------------------------------------+
| timestampdiff(YEAR, '2002-05-01 00:00:00', '2001-01-01 00:00:00') |
+-------------------------------------------------------------------+
| -1 |
+-------------------------------------------------------------------+
MySQL > SELECT TIMESTAMPDIFF(MINUTE,'2003-02-01','2003-05-01 12:05:55');
+---------------------------------------------------------------------+
| timestampdiff(MINUTE, '2003-02-01 00:00:00', '2003-05-01 12:05:55') |
+---------------------------------------------------------------------+
| 128885 |
+---------------------------------------------------------------------+
MySQL > SELECT TIMESTAMPDIFF(MILLISECOND,'2003-02-01','2003-05-01');
+--------------------------------------------------------+
| timestampdiff(MILLISECOND, '2003-02-01', '2003-05-01') |
+--------------------------------------------------------+
| 7689600000 |
+--------------------------------------------------------+
