SQL中datediff函数如何用？（代码详解）

本文供给了SQLServer中的DATEDIFF()函数的运用示例。DATEDIFF()函数语法如下:DATEDIFF(datepart,startdate,enddate) 在SQL Server中，可以运用T-SQL DATEDIFF()函数返回两个日期之间的悬殊。它适用于任何可以解析为time、date、smalldatetime、datetime、datetime2或datetimeoffset值的表达式。因而，你也可以得到两次的差值。

+----------+
| Result   |
|----------|
| 365      |
+----------+

示例2

下面是另一个例子，我声明了两个变量并为它们分配了两个不一样的日期(我运用DATEADD()将首先个日期增加1年)。然后运用DATEDIFF()返回该日期的各个dateparts:

DECLARE @date1 datetime2 = '2000-01-01 00:00:00.0000000';
DECLARE @date2 datetime2 = DATEADD(year, 1, @date1);
SELECT 
    DATEDIFF( year, @date1, @date2 ) AS Years,
    DATEDIFF( quarter, @date1, @date2 ) AS Quarters,
    DATEDIFF( month, @date1, @date2 ) AS Months,
    DATEDIFF( week, @date1, @date2 ) AS Weeks,
    DATEDIFF( dayofyear, @date1, @date2 ) AS DayOfYear,
    DATEDIFF( day, @date1, @date2 ) AS Days;

效果：

+---------+------------+----------+---------+-------------+--------+
| Years   | Quarters   | Months   | Weeks   | DayOfYear   | Days   |
|---------+------------+----------+---------+-------------+--------|
| 1       | 4          | 12       | 53      | 366         | 366    |
+---------+------------+----------+---------+-------------+--------+

示例3

如前所述，你还可以返回日期之间的工夫局部。下面是返回日期/工夫值之间的小时、分钟和秒数的例子:

DECLARE @date1 datetime2 = '2000-01-01 00:00:00.0000000';
DECLARE @date2 datetime2 = DATEADD(hour, 1, @date1);
SELECT 
    DATEDIFF( hour, @date1, @date2 ) AS Hours,
    DATEDIFF( minute, @date1, @date2 ) AS Minutes,
    DATEDIFF( second, @date1, @date2 ) AS Seconds;

效果：

+---------+-----------+-----------+
| Hours   | Minutes   | Seconds   |
|---------+-----------+-----------|
| 1       | 60        | 3600      |
+---------+-----------+-----------+

示例4

下面是一个猎取两个日期/工夫值之间的毫秒、微秒和纳秒数的例子:

DECLARE @date1 datetime2 = '2000-01-01 00:00:00.0000000';
DECLARE @date2 datetime2 = DATEADD(millisecond, 1, @date1);
SELECT    
    DATEDIFF( millisecond, @date1, @date2 ) AS Milliseconds,
    DATEDIFF( microsecond, @date1, @date2 ) AS Microseconds,
    DATEDIFF( nanosecond, @date1, @date2 ) AS Nanoseconds;

效果：

+----------------+----------------+---------------+
| Milliseconds   | Microseconds   | Nanoseconds   |
|----------------+----------------+---------------|
| 1              | 1000           | 1000000       |
+----------------+----------------+---------------+

示例5 - Error差错!

要是你尝试做一些极度的事情，比方，返回100年后的纳秒数，你会得到一个差错。这是由于DATEDIFF()返回一个int值，100年内的纳秒数比int数据类型能够处置的纳秒数还要多。

要是你试着这么做会产生什么呢?

DECLARE @date1 datetime2 = '2000-01-01 00:00:00.0000000';
DECLARE @date2 datetime2 = DATEADD(year, 100, @date1);
SELECT    
    DATEDIFF( millisecond, @date1, @date2 ) AS Milliseconds,
    DATEDIFF( microsecond, @date1, @date2 ) AS Microseconds,
    DATEDIFF( nanosecond, @date1, @date2 ) AS Nanoseconds;

效果：

The datediff function resulted in an overflow. The number of dateparts separating two date/time instances is too large. Try to use datediff with a less precise datepart.

固然要是你真的必需寻出100年内有多少纳秒，那么可以运用DATEDIFF_BIG()函数。这个函数返回一个带符号的bigint数据类型，它允许你返回比DATEDIFF()更大的值。