Number of Mondays between two dates

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Number of Mondays between two dates

January 15, 2010 at 7:40 am

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Jesper-244176 SSCertifiable Points: 7142 More actions · Answer 1

Sergiy, I believe the following is correct:

select case when

Datediff(dd, @startDate, @endDate - (@@Datefirst+Datepart(dw,@endDate)-2) %7) < 0

then 0 else

Datediff(dd, @startDate, @endDate - (@@Datefirst+Datepart(dw,@endDate)-2) %7) / 7 + 1

end

The division and modulo operations behave strangely for negative numbers, I think. E.g

select (-1)/7

select (-1) % 7

is 0 and -1, respectively. I would expect it to be -1 and 6, respectively, and then your posted formula would work.

Sergiy SSC Guru Points: 110209 More actions · Answer 2

There is no way to get (-1)%7 in my formula.

Both @@Datefirst and Datepart(dw,@endDate) are positive values, their SUM is >= 2.

And for both dtes '2005-12-31' it returns 0 which is right - no Mondays between Saturday 31/12/2005 and Saturday 31/12/2005.

But one thing I missed when copied script here:

@StartDate must be shifted to Previous Monday as well:

select Datediff(dd,

@StartDate - (@@Datefirst+Datepart(dw,@StartDate)-2) %7,

@EndDate - (@@Datefirst+Datepart(dw,@EndDate)-2) %7)

/ 7

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Code for TallyGenerator

Jesper-244176 SSCertifiable Points: 7142 More actions · Answer 3

What I meant was that you could easily get (-1)/7, although this wasn't clear from what I wrote.

I assumed you wanted to add 1 to the formula you stated perviously. Anyway, it seems to return 0 whenever the two dates are the same.

I think this is also the case with your new formula - you always get 0 when the two dates are the same

Sergiy SSC Guru Points: 110209 More actions · Answer 4

I mean get (-1)%7, not get (-1)/7

And what you expect to get when 2 dates are the same?

There are no Mondays between them, so it must be 0.

What's wrong with that?

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Code for TallyGenerator

Jesper-244176 SSCertifiable Points: 7142 More actions · Answer 5

I see. No, nothing's wrong. You include the second date but not the first. I thought you included both dates (like I do in my query). I apologize, your query works perfectly.

But... I think my query is slightly faster (replace '19000102' by '19000101' to get the same output as your query).

Sergiy SSC Guru Points: 110209 More actions · Answer 6

My query does not depend on DATEFIRST settings.

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Code for TallyGenerator

Jesper-244176 SSCertifiable Points: 7142 More actions · Answer 7

Jesper-244176

SSCertifiable

Points: 7142

February 18, 2006 at 5:40 am

#621880

So I've noticed - neither does mine

Sergiy SSC Guru Points: 110209 More actions · Answer 8

What about server collation?

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Code for TallyGenerator

Sergiy SSC Guru Points: 110209 More actions · Answer 9

And you formula returns 1 Monday between '2005-02-19' and '2006-02-20' and between '2006-02-20' and '2006-02-21'.

This Monday is included in both periods. Sounds not right.

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Code for TallyGenerator

Jesper-244176 SSCertifiable Points: 7142 More actions · Answer 10

My formula

datediff(d, '19000101', @dateTo)/7 - datediff(d, '19000102', @dateFrom)/7

doesn't depend on datefirst settings and server collation. Like I have said before, it returns the number of Mondays between @dateFrom and @dateTo, both days inclusive. '2006-02-20' is a Monday, as far as I know, and therefore it will return 1 for both periods.

If you insist on not including the first date (but still including the second date), my formula is easily modified:

datediff(d, '19000101', @dateTo)/7 - datediff(d, '19000101', @dateFrom)/7

I have tested that this formula has the same output as your formula for every single day of the year 2005 (365*365 test cases), so I guess both are correct. My formula, however, performs slightly better than your formula - at least, that was what I concluded in my test.

Sergiy SSC Guru Points: 110209 More actions · Answer 11

Sorry, I was looking on another your formula.

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Code for TallyGenerator

Jesper-244176 SSCertifiable Points: 7142 More actions · Answer 12

Yes, the first one wasn't correct for dates after '20060213', as Jeff Moden pointed out. My present formula isn't correct for dates before '19000101', but I guess that's all right

g.fichtner Valued Member Points: 55 More actions · Answer 13

Hi - great posting and answers... I realize the posts are quite old - but hoping some of you experts are still available out here.

I used the counting method in crystal reports and it seems pretty good- but I am intrigued by the formula. But - when I try the formula I do not get the same results.

I am using the date range 2009-03-01 to 2009-03-31. Should have 5 Sundays, Mondays, Tuesdays and 4 of the rest of the week days.

When I use the formula

cnt:=datediff("d", date(1900,01,01), dt2)/7 - datediff("d", date(1900,01,02), dt1)/7

I show 4.43. Does your formula assume it needs to be rounded up? I tried to run it in my sql plus - but it did not like "DATEDIFF". I ran a modified version (just subtracting the dates) - and it also yielded a fractional number -

1 SELECT

2 (TO_DATE('03/31/2009','MM/DD/YYYY')-TO_DATE('01/01/1900','MM/DD/YYYY'))/7 -

3 (TO_DATE('03/01/2009','MM/DD/YYYY')-TO_DATE('01/01/1900','MM/DD/YYYY'))/7

4 datediff

5* FROM dual

SQL> /

DATEDIFF

----------

4.28571429

SQL>

====

Am I missing something. Were the divisions supposed to be truncated maybe?

thanks in advance for your help.

updated - tried with

cnt:=truncate(datediff("d", date(1900,01,01), dt2)/7) -

truncate(datediff("d", date(1900,01,02), dt1)/7)

and intitial testing looks like that may be working (o - i only tested one thing - hopefully this is right....)

Greg

Offel Newbie Points: 1 More actions · Answer 14

A different approach is reccursion: (Im using T-sql)

DECLARE @startDate date = '2019-04-01',
@endDate date = '2019-05-01'

;WITH cte AS (
SELECT @startDate as date_
UNION ALL
SELECT CAST(DATEADD(day,1,date_) as date)
FROM cte
WHERE date_ < @endDate
)

SELECT sum(case when datepart(dw,date_) = 2 then 1 else 0 end) as nrOfMondays
FROM cte