Get correct week number for december 31st - sql-server

Question

In SQL Server, I am trying to get the week number (with the year) of a given date, European style, so I'm using DATEPART with ISO_WEEK argument:
SELECT CAST(DATEPART(year, myDate) AS VARCHAR) + RIGHT('0' + CAST(DATEPART(ISO_WEEK, myDate) AS VARCHAR), 2);
This works well, except for December 31st of 2018, which falls on the 1st week of 2019, but since I'm using DATEPART for the year and the week separately, this obviously can't work. Here's an example when myDate is 31-12-2018:
SELECT CAST(DATEPART(year, '31-12-2018') AS VARCHAR) + RIGHT('0' + CAST(DATEPART(ISO_WEEK, '31-12-2018') AS VARCHAR), 2);
The above query returns '201801'.
Is there a way to simply get 201901 for December 31st 2018, without testing explicitly for this date ?

Answer 1

See the possible duplicate reference and a little tidy up:
declare #d datetime = '2018-12-31';
select cast(year(dateadd(day, 26 - datepart(ISO_WEEK, #d), #d)) as varchar(4)) + right(('0' + cast(datepart(ISO_WEEK, #d) as varchar(2))), 2)
results in 201901.
Test code:
select cast(ds.d as date) as 'date', cast(year(dateadd(day, 26 - datepart(ISO_WEEK, ds.d), ds.d)) as varchar(4)) + right(('0' + cast(datepart(ISO_WEEK, ds.d) as varchar(2))), 2) as "IsoWeek"
from (
select dateadd(day, x.num, '2018-12-30') as d
from (
select h.num + d.num + s.num as num
from (
select 0 num union select 100 num union select 200 num union select 300 num
) h
cross join (
select 0 num union select 10 num union select 20 num union select 30 num
) d
cross join (
select 0 num union select 1 num union select 2 union select 3 num
) s
) x
) ds
order by ds.d
which results in:
date IsoWeek
2018-12-30 201852
2018-12-31 201901
2019-01-01 201901
2019-01-02 201901
2019-01-09 201902
2019-01-10 201902
2019-01-11 201902
2019-01-12 201902
2019-01-19 201903
2019-01-20 201903
2019-01-21 201904
2019-01-22 201904
2019-01-29 201905
2019-01-30 201905
2019-01-31 201905
2019-02-01 201905
2019-04-09 201915
2019-04-10 201915
2019-04-11 201915
2019-04-12 201915
2019-04-19 201916
2019-04-20 201916
2019-04-21 201916
2019-04-22 201917
2019-04-29 201918
2019-04-30 201918
2019-05-01 201918
2019-05-02 201918
2019-05-09 201919
2019-05-10 201919
2019-05-11 201919
2019-05-12 201919
2019-07-18 201929
2019-07-19 201929
2019-07-20 201929
2019-07-21 201929
2019-07-28 201930
2019-07-29 201931
2019-07-30 201931
2019-07-31 201931
2019-08-07 201932
2019-08-08 201932
2019-08-09 201932
2019-08-10 201932
2019-08-17 201933
2019-08-18 201933
2019-08-19 201934
2019-08-20 201934
2019-10-26 201943
2019-10-27 201943
2019-10-28 201944
2019-10-29 201944
2019-11-05 201945
2019-11-06 201945
2019-11-07 201945
2019-11-08 201945
2019-11-15 201946
2019-11-16 201946
2019-11-17 201946
2019-11-18 201947
2019-11-25 201948
2019-11-26 201948
2019-11-27 201948
2019-11-28 201948

Answer 2

the problem is that there is a unique case where the week number can be very 'early' in the year, although it is actually month 12, therefore it is easy to check if there is a start of year week in December, and in that case simply add one to the year,
DECLARE #mydate as datetime2 = '20270101';
SELECT CAST(DATEPART(year, #myDate)
+ case when DATEPART(ISO_WEEK, #myDate) < 2 and month(#mydate) =12 then
1
else
0
end
+ case when DATEPART(ISO_WEEK, #myDate) >10 and month(#mydate) =1 then
-1
else
0
end
AS VARCHAR)
+ RIGHT('0' + CAST(DATEPART(ISO_WEEK, #myDate) AS VARCHAR), 2);
I added a correction for the 2027 problem highlighted in another answer.

Answer 3

Check following query, its using last day of the week to get the year value
SELECT CAST(DATEPART(year, DATEADD(wk, 1, DATEADD(DAY, 0-DATEPART(WEEKDAY, '2018-12-31'), DATEDIFF(dd, 0, '2018-12-31')))) AS VARCHAR) + RIGHT('0' + CAST(DATEPART(ISO_WEEK, '2018-12-31') AS VARCHAR), 2);