为什么不同的日期数学运算产生相同的值？

Question

我可以将日期加六天并得到我期望的结果，但如果我将同一日期加七天，我会得到相同的日期。

我的意思是这个；当用户输入“12/27/2015”作为@BegDate 参数的值时，此声明：

DECLARE 
@Week1End datetime = DATEADD(Day, 6, @BegDate),
@Week2Begin datetime = DATEADD(Day, 7, @BegDate);

...结果@Week1End 和@Week2Begin 都是同一日期 (2016-01-02)，从返回的结果集中可以看出：

这是 TSQL：

CREATE PROCEDURE [dbo].[variancePriceByProductWithPriceChangePercentage]
    @Unit varchar(25),
    @BegDate datetime,
    @EndDate datetime
AS

DECLARE 
@Week1End datetime = DATEADD(Day, 6, @BegDate),
@Week2Begin datetime = DATEADD(Day, 7, @BegDate);

Select Ind.Description,
    @BegDate  BegDate,
    @Week1End Week1End,
    @Week1End Week2Begin,
    @EndDate EndDate,
    SUM(CASE WHEN Ind.InvoiceDate BETWEEN @BegDate AND @Week1End THEN Ind.QtyShipped ELSE 0 END) 
Week1Usage,
    SUM(CASE WHEN Ind.InvoiceDate BETWEEN @Week2Begin AND @EndDate THEN Ind.QtyShipped ELSE 0 END) 
Week2Usage,
    SUM(CASE WHEN Ind.InvoiceDate BETWEEN @BegDate AND @Week1End THEN Ind.Price ELSE 0 END) 
Week1Price,
    SUM(CASE WHEN Ind.InvoiceDate BETWEEN @Week2Begin AND @EndDate THEN Ind.Price ELSE 0 END) -
   SUM(CASE WHEN Ind.InvoiceDate BETWEEN @BegDate AND @Week1End THEN Ind.QtyShipped ELSE 0 END) 
UsageVariance,
    SUM(CASE WHEN Ind.InvoiceDate BETWEEN @Week2Begin AND @EndDate THEN Ind.QtyShipped ELSE 0 END) 
-
    SUM(CASE WHEN Ind.InvoiceDate BETWEEN @BegDate AND @Week1End THEN Ind.Price ELSE 0 END) 
PriceVariance,
    (SUM(CASE WHEN Ind.InvoiceDate BETWEEN @Week2Begin AND @EndDate THEN Ind.QtyShipped ELSE 0 
END)  -
    SUM(CASE WHEN Ind.InvoiceDate BETWEEN @BegDate AND @Week1End THEN Ind.Price ELSE 0 END) )
    / 
NULLIF(SUM(CASE WHEN Ind.InvoiceDate BETWEEN @BegDate AND @Week1End THEN Ind.Price ELSE 0 END),0) 
PercentageOfPriceVariance
    From    InvoiceDetail Ind
    Where   Ind.Unit = @Unit
    AND Ind.InvoiceDate BETWEEN @BegDate AND @EndDate
    Group By Ind.Description

那么为什么 @BegDate + 6 等同于 @BegDate + 7

Answer 1

你确定吗：

DECLARE @BegDate datetime = GETDATE()

SELECT @BegDate, DATEADD(Day, 6, @BegDate), DATEADD(Day, 7, @BegDate);

结果：

我认为你的问题出在这里：

@Week1End Week1End,
@Week1End Week2Begin, ---<<<<<---

应该是：

@Week1End Week1End,
@Week2BeginWeek2Begin, ---<<<<<---

Answer 2

@Week1End Week1End,
@Week1End Week2Begin,

再看看这段代码...

为什么不同的日期数学运算产生相同的值？

Why do different date math operations produce the same value?

sql-server

tsql

date-math