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Date query -- to find a day from 3 weeks prior

Hi Experts,

I need your help in writing a date query to find out friday's date from 3 weeks prior when I run it for current date. And if the current date is friday then I need to get friday's date from 2 weeks prior.

For Example:

If I run the query for date: 2013-06-12, I should get the date: 2013-05-24.

If I run the query for date: 2013-06-14, I should get the date: 2013-05-31.

Please help!!!

Thanks a lot!!
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ravichand-sql
Asked:
ravichand-sql
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3 Solutions
 
lluddenCommented:
Find the next Friday after the current date (non-inclusive), then go back 3 weeks

SET DATEFIRST 1
DECLARE @DateToTest DATE = '20130614'
SELECT dateadd(day,-21,dateadd(day, CASE WHEN datepart(dw,@DateToTest) < 5 THEN 5 - datepart(dw, @DateToTest) ELSE 12 - datepart(dw,@DateToTest) END, @DateToTest))

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lluddenCommented:
Or for the default datefirst

SET DATEFIRST 7
DECLARE @DateToTest DATE = '20130612'
SELECT dateadd(day,-21,dateadd(day, CASE WHEN datepart(dw,@DateToTest) < 6 THEN 6 - datepart(dw, @DateToTest) ELSE 13 - datepart(dw,@DateToTest) END, @DateToTest))

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PortletPaulCommented:
this approach uses modulus 7 and is independent of the datefirst (or any other) dbms setting(s)
declare @dateFrom as datetime
set @dateFrom = getdate() -- normally this would be used
--set @dateFrom = '2013-06-12' -- test 1
--set @dateFrom = '2013-06-14' -- test 2

set @datefrom = case when datediff(day,0,@dateFrom) % 7 = 4 then dateadd(day,-14,@dateFrom)
                     else dateadd(day,-21+(datediff(day,0,@dateFrom) % 7 ),@dateFrom) 
                end

select @dateFrom, datediff(day,0,@dateFrom) % 7

--test 1: May, 24 2013
--test 2: May, 31 2013

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PortletPaulCommented:
oh, 0 is always Monday using this modulus approach, 4 is always Friday
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PortletPaulCommented:
I checked my results just now and found it was in error. This was due to me wondering what you wanted if getdate() was Sat/Sun and here I assume you would want the same results as a getdate() of Friday. So a revised approach
tests:
--set @dateFrom = '2013-06-10' -- test Mon 0
--set @dateFrom = '2013-06-11' -- test Tue 1
--set @dateFrom = '2013-06-12' -- test Wed 2
--set @dateFrom = '2013-06-13' -- test Thu 3
--set @dateFrom = '2013-06-14' -- test Fri 4
--set @dateFrom = '2013-06-15' -- test Sat 5
--set @dateFrom = '2013-06-16' -- test Sun 6
results:
-- Monday            0      2013.05.24      Friday
-- Tuesday            1      2013.05.24      Friday
-- Wednesday      2      2013.05.24      Friday
-- Thursday         3      2013.05.24      Friday
-- Friday               4      2013.05.31      Friday
-- Saturday          5      2013.05.31      Friday
-- Sunday             6      2013.05.31      Friday
from this code:
declare @dateFrom as datetime
set @dateFrom = getdate() -- normally this would be used

set @datefrom = case when datediff(day,0,@dateFrom) % 7 >= 4 then dateadd(day,-10-(datediff(day,0,@dateFrom) % 7 ),@dateFrom)
                     else dateadd(day,-17-(datediff(day,0,@dateFrom) % 7 ),@dateFrom) 
                end

select convert(varchar, @dateFrom ,102), datename(weekday,@datefrom)

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http://sqlfiddle.com/#!6/59e7c/1
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PortletPaulCommented:
Happy you have a solution.

A should be the default grade awarded unless the answer is deficient.

How was the answer deficient?
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ravichand-sqlAuthor Commented:
Hi Paul,

Im sorry.. I did not know this. Please let me know if I can change the grade for this question. I will make sure I will grade appropriately from the next question.

Thanks!!!
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PortletPaulCommented:
You may request attention for the question to be re-opened, and then amend.

Thanks for sharing out the points by the way - I believe that is a good thing to do when there are multiple solutions (but you are effectively also marking me down by awarding twice to variants of the same solution).
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