Pull data from the third week back from this week? [duplicate] - sql

Question

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Query to get content older than 3 weeks
(3 answers)
Closed 2 years ago.
This week is the week of 12/28/20. I want to pull data from the third week back of whatever the current week is. So if I run my query today I want it to pull data for the week of 12/7 only. I'm trying to do this and its not returning any results. Any ideas? Thank you
WHERE date = DATEADD(week, -3, GETDATE())

Answer 1

I prefer the following which:
Allows you to use the Date column without calling a function on it. This is important for sargability i.e. the ability for SQL Server to use an index on the Date column. Although in my testing a cast to date still did you use the index, its best practice to avoid functions calls on the columns in where clauses.
Avoids the use of between, which I always have to take a moment to think about because its >= the first condition and <= the second condition. So its easy to get the logic wrong and include 8 days instead of 7. Instead I always use an explicit compare so its obvious what the logic is.
Note the >= and < now rather than <=.
where [Date] >= convert(date,dateadd(week, -3, current_timestamp))
and [Date] < convert(date,dateadd(week, -2, current_timestamp))
Now
Start Of Week (Inc)
End Of Week (Ex)
2020-12-29 20:07:16.373
2020-12-08
2020-12-15
And if you need it to start on your week start day then use:
where [Date] >= convert(date,dateadd(day, (-1*datepart(weekday,current_timestamp))+1, dateadd(week, -3, current_timestamp)))
and [Date] < convert(date,dateadd(day, (-1*datepart(weekday,current_timestamp))+1, dateadd(week, -2, current_timestamp)))
Now
Start Of Week (Inc)
End Of Week (Ex)
2020-12-29 20:07:16.373
2020-12-06
2020-12-13

Answer 2

That is only going to give you data that has a date that was three weeks ago from the current time to the nearest thousandth of a second. It's fairly unlikely that you have any data that exactly matches that.
What I expect you are after is data that is between three and two weeks ago which can be obtained like this:
WHERE date BETWEEN DATEADD(week, -3, GETDATE()) AND DATEADD(week, -2, GETDATE())
However if you run that half way through a day, it will only give you data from half way through the day three weeks ago until half way through the day two weeks ago. You probably want to go from midnight at the start and end of the day. This can be done by casting everything to the DATE type, which has the effect of discarding the time, like this:
WHERE CAST(date AS DATE) >= CAST(DATEADD(week, -3, GETDATE()) AS DATE) AND CAST(date AS DATE) < CAST(DATEADD(week, -2, GETDATE()) AS DATE)
It is also possible that you want to have the weeks always start on the same day of the week. This can be done using the DATEPART function.
WHERE DATEPART(week, date) = DATEPART(week, DATEADD(week, -3, GETDATE())) AND DATEPART(year, date) = DATEPART(year, DATEADD(week, -3, GETDATE()))
Depending upon what you want and how your database is set up this last option may require setting the SET DATEFIRST setting to get the correct day to start the week on. For example to start the week on Mondays use:
SET DATEFIRST 1;