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 How to use Date Diff Function?

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Starting Member

6 Posts

Posted - 2015-05-02 : 03:00:20
I have added one webpage designed in ASP.Net with C# and sql server 2005 as database. There is table for user registration in which there is a column for user creation date time the data type of that column is date time .
I would like to fetch data of those user who have created profile within 7 days. For getting desired result I am trying this query.

select Name ,Profession,ProfileCreationDate from tblRegistration where DATEDIFF ( Day , '" + System.DateTime.Now + "',ProfileCreationDate)<7 order by ProfileCreationDate DESC

System.DateTime.Now is a function for getting current date time in C#

The query is neither giving error nor giving desired result please help me.


22859 Posts

Posted - 2015-05-02 : 03:52:52
I would have done

WHERE ProfileCreationDate >= DATEADD(Day, -7, GetDate())

(GetDate() is a SQL function for current Date AND Time - so that will all happen at the SQL end, rather than based on the Web Server time. That only matters if the time is different on the Web Server and the SQL Server!! If the date/time for ProfileCreationDate is automatically added by SQL then I would user SQL time, if it is added by your APP instead then maybe I would use Web Server time. What if one is on Daylight Saving Time and the other not? Or the servers are in different countries?? Anyway, that may not be a problem you need to worry about!!!!)

Next problem is that DATEADD (and DATEDIFF) will add a DAY but keep the TIME part the same ... when you say "Last 7 days" I expect you mean "Any records in the last 7 days and today, so far" and that means starting at midnight 7 days ago.

For example if I do

SELECT GetDate() AS TimeNow,
DATEADD(Day, -7, GetDate()) AS TimeBefore

I get

TimeNow TimeBefore
----------------------- -----------------------
2015-05-02 08:50:03.893 2015-04-25 08:50:03.893

There are various ways of getting "Midnight" on a date, usually involving string manipulation, but this is the most efficient for SQL as it only uses integer maths, no [slower] string conversion

SELECT GetDate() AS TimeNow,
DATEADD(Day, 0, DATEDIFF(Day, 0, GetDate()) - 7) AS TimeBefore

TimeNow TimeBefore
----------------------- -----------------------
2015-05-02 08:52:22.313 2015-04-25 00:00:00.000

so if your APP will use the Date/Time on the SQL Server then your code would need to be:

select Name ,Profession,ProfileCreationDate from tblRegistration
where ProfileCreationDate >= DATEADD(Day, 0, DATEDIFF(Day, 0, GetDate()) - 7)
order by ProfileCreationDate DESC
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