datediff
description
Syntax
DATETIME DATEDIFF(DATETIME expr1,DATETIME expr2)
计算expr1 - expr2，结果精确到天。
expr1 和 expr2 参数是合法的日期或日期/时间表达式。
注释：只有值的日期部分参与计算。
example
mysql> select datediff(CAST('2007-12-31 23:59:59' AS DATETIME), CAST('2007-12-30' AS DATETIME));
+-----------------------------------------------------------------------------------+
| datediff(CAST('2007-12-31 23:59:59' AS DATETIME), CAST('2007-12-30' AS DATETIME)) |
+-----------------------------------------------------------------------------------+
| 1 |
+-----------------------------------------------------------------------------------+
mysql> select datediff(CAST('2010-11-30 23:59:59' AS DATETIME), CAST('2010-12-31' AS DATETIME));
+-----------------------------------------------------------------------------------+
| datediff(CAST('2010-11-30 23:59:59' AS DATETIME), CAST('2010-12-31' AS DATETIME)) |
+-----------------------------------------------------------------------------------+
| -31 |
+-----------------------------------------------------------------------------------+keywords
DATEDIFF