SQL - pick current date from machine and compare with previous week - sql

Question

For the below query
Sum(CASE WHEN dbo.sales.date BETWEEN '2016-07-17' AND '2016-07-23' THEN dbo.sales.sellinc END) AS ActualSales
Instead of hard coding the date. I would like to pick current date from machine and compare
Sum(CASE WHEN dbo.sales.date BETWEEN '**previous week (31 week 2016)**' AND '**Last year same week (31 week 2015)**' THEN dbo.sales.sellinc END) AS ActualSales
Week start from Sunday and ends Saturday. Any help please?

Answer 1

I think this snippet might help you to get a range of one year from the previous week to last year.
Sum(CASE
WHEN dbo.sales.date
BETWEEN DATEADD(YEAR, -1, DATEADD(WEEK, -1, GETDATE())) AND
DATEADD(WEEK, -1, GETDATE()) THEN dbo.sales.sellinc END) AS ActualSales
You can also modify the logic if you want to consider Friday as an exception.
In that case, you have to use DATEPART(WEEKDAY, GETDATE()) function to get the week day which returns 1 for Sunday and 7 for Saturday. It would be prettier if you move the date generation logic to a variable and then use it to select your data.

Answer 2

Others have already noted that your method of week numbering is going to impact your solution. Here's one that just looks for the Sunday of the current week and then subtracts 52 weeks. (You could also just deduct 364 days in the previous step and use dateadd only once.) So the correspondence with the previous year's week is essentially the nearest calendar date falling on Sunday that year.
with dates as (
select
dateadd(week, -52,
dateadd(day,
1 - datepart(weekday, getdate()), /* Sunday of current week */
cast(getdate() as date)
)
) as WeekStartY-1,
dateadd(day, 1 - datepart(weekday, cast(getdate() as date)) as WeekStartY-0
)
select
sum(case when cast(s."date" as date)
between d.WeekStartY-0 and dateadd(day, 6, WeekStartY-0) then s.sellinc end
) as ActualSalesY-0,
sum(case when cast(s."date" as date)
between d.WeekStartY-1 and dateadd(day, 6, WeekStartY-1) then s.sellinc end
) as ActualSalesY-1,
from dbo.Sales s cross apply dates d;
This can occasionally produce an oddity where the weeks both start in the same year. It happens when the year ends on Sunday, or Monday in leap years, as with 2012. So Y-1 for December 30, 2012 was January 1, 2012. But consider that most of the days for that week actually fall in 2013 and the correspondence makes sense from that perspective.

Answer 3

You can try using DATEADD:
SUM(CASE WHEN dbo.sales.date BETWEEN DATEADD(week, 30, '2016/01/01') AND
DATEADD(week, 30, '2015/01/01')
THEN dbo.sales.sellinc END) AS ActualSales
This will give you the one year range between the 30th week of 2015 and 2016, using the SQL Server week, whose first week begins on January 1st, regardless of the day. If you really want to use the ISO week, then it will be a lot more work.