t-sql中两个日期相减

Subtraction of two dates in t-sql(t-sql中两个日期相减)

问题描述

我正在阅读前任编写的脚本.

I was going through a script written by a predecessor.

谁能给我解释一下为什么会有这个说法

Can someone explain to me why would this statement

--- CreatedDateTime is a datetime column in SALES_ORDER table.
SELECT * FROM SALES_ORDER
WHERE GETDATE() - CreatedDateTime < 1

返回与

SELECT * FROM SALES_ORDER 
WHERE DateDiff(hh,CreatedDateTime, GetDate()) < 24

推荐答案

从 DATETIME 中减去 数字 记录在 此处:还可以从日期中减去一个数字，以天为单位."

Subtraction of a number from a DATETIME is documented here: "Can also subtract a number, in days, from a date."

declare @Now as DateTime = GetDate();
declare @OneWeekAgo as SQL_Variant = @Now - 7;

select @Now as [Now], @OneWeekAgo as [Delta], SQL_Variant_Property( @OneWeekAgo, 'BaseType' ) as [Data Type];

在使用带有日期和时间数据的运算符类型:要对所有日期和时间数据类型进行加减运算，请使用 DATEADD 和 DATEDIFF."

Under Using Operators with Date and Time Data Types: "To add and subtract for all date and time data types, use DATEADD and DATEDIFF."

在可能违反最小惊讶原则的情况下，我们看到了以下奇怪的结果:

In a possible violation of the Principle of Least Astonishment we see the following curious result:

declare @Now as DateTime = GetDate();
declare @Then as DateTime = '17760704';
declare @Delta as SQL_Variant = @Now - @Then;

select @Now as [Now], @Then as [Then], @Delta as [Delta],
  SQL_Variant_Property( @Delta, 'BaseType' ) as [Data Type],
  Cast( @Delta as Int ) as [Days];

<小时>

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