爲什麼不同日期的數學運算產生相同的值？

Question

我可以添加六天到一個日期，並得到我的期望，但如果我添加七天到同一日期，我得到相同的日期。

我的意思是這個;當用戶輸入「2015年12月27日」作爲@BegDate參數的值，這個聲明：

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

...結果按@ Week1End和@ Week2Begin是相同的日期（2016 -01-02），你可以從返回的結果集看到：

這是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 

所以爲什麼@BegDate + 6等同於同樣的事情@BegDate + 7

Answer 1

@Week1End Week1End, 
@Week1End Week2Begin, 

看看這塊再次代碼...

Answer 2

你肯定：

DECLARE @BegDate datetime = GETDATE() 

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

結果：

我覺得你的問題在這裏：

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

它應該是：

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