Datediff 函数（）给我错误的答案

Question

I am using SELECT DATEDIFF(DD,$StartDate, $Today) to calculate the number of days between 2 dates. $$StartDate is 2020-11-19 and $Today is current date ( $Today = date("Ymd"); ). I am getting 28 days as the number of days which is not correct. What am I doing wrong?

Answer 1

To answer the actual question of why the value is "wrong" it's because it isn't ; 28 is correct for the values you have passed. The problem is you aren't parametrising. DATEDIFF is actually working completely correctly. What you are doing is the following:

SELECT DATEDIFF(DD,2020-11-19, 2021-02-01);

That returns 28 . That is correct . What you have is a synonym of the following expressions as well:

SELECT DATEDIFF(DD,1990,2018);
SELECT DATEDIFF(DD,'19050614','19070712');
--and even
SELECT 2018 - 1990;

Because you aren't properly parametrising (and injecting) the value you are passing isn't a datetime value, it's an expression of int s . Parametrise your query, and pass a date and time date type value, and the SQL works perfectly correctly. For example:

--This wouldn't be in your SQL, but something equivilent would happen when the parameters are pass from your application
DECLARE @StartDate date = '20201119',
        @Today date = '20210201';
--And the DATEDIFF function
SELECT DATEDIFF(DAY,@StartDate, @Today);

This also returns the correct value for the expression, but also the value you likely expect, 74 .