sql server - Number of weeks and partial weeks between two days calculated wrong

Question

Why is this returning 4 instead of 6?

SET DATEFIRST 1
SELECT DATEDIFF(WEEK, CAST('2017-01-01' AS DATE), CAST('2017-01-31' AS DATE))

Is week as datepart calculating only weeks from monday - sunday (whole weeks)? How to get all weeks - including those which doesn't have seven days ? In the case above answer should be 6.

Answer 1

DATEDIFF counts transitions, not periods (e.g. look at DATEDIFF(year,'20161231','20170101')). It also treats Sunday as the first day of the week. So, how do we compensate for these features? First, we shift our dates so that Mondays are the new Sundays, and second we add 1 to compensate for the Fence-Post error:

declare @Samples table (
    StartAt date not null,
    EndAt date not null,
    SampleName varchar(93) not null
)
insert into @Samples (StartAt,EndAt,SampleName) values
('20170101','20170131','Question - 6'),
('20170102','20170129','Exactly 4'),
('20170102','20170125','3 and a bit, round to 4'),
('20170101','20170129','4 and 1 day, round to 5')
--DATEDIFF counts *transitions*, and always considers Sunday the first day of the week
--We subtract a day from each date so that we're effectively treating Monday as the first day of the week
--We also add one because DATEDIFF counts transitions but we want periods (FencePost/FencePanel)
select *,
    DATEDIFF(WEEK, DATEADD(day,-1,StartAt), DATEADD(day,-1,EndAt)) +1
    as NumWeeks
from @Samples

Results:

StartAt    EndAt      SampleName                 NumWeeks
---------- ---------- -------------------------- -----------
2017-01-01 2017-01-31 Question - 6               6
2017-01-02 2017-01-29 Exactly 4                  4
2017-01-02 2017-01-25 3 and a bit, round to 4    4
2017-01-01 2017-01-29 4 and 1 day, round to 5    5

If this doesn't match what you want, perhaps you can adopt and adapt my @Samples table to show the results you do expect.