Q:

How to properly convert a date into a ISO-8601 Week number format in SQL Server?

ISO-8601 states that week numbers are to be formated as YYYY-W## - observe that the week number should be two digits as 01, 02, ...

SELECT cast(DATEPART(YYYY, CreationDate) as varchar) + '-W' + 
       cast(DATEPART(ISO_WEEK, GETDATE())`

The problem is that this gives the week number as 1, 2, ...
What is the correct way of extracting 2020-W01, ...

A:

The original question text relates to a simple formatting issue of the ISO_WEEK number, but I feel there is a bigger issue here with the concatenation of the year part. Simply concatenating DatePart YYYY and ISO_WEEK will yield incorrect (or at least unexpected) results for days in week 1 and/or 53. Notably dates like 2014-12-31 are part of week 1, but not 2014-W01. It is part of 2015-W01. Similarly, 2016-01-01 will be part of 2015-W53, not 2016-W53. In order to determine the Iso Year-Week the year must be corrected to take this into account:
With
    Dates (Date) As (
            Select Convert( date, N'2014-12-31' )
Union All   Select Convert( date, N'2015-01-01' )
Union All   Select Convert( date, N'2015-12-31' )
Union All   Select Convert( date, N'2016-01-01' )
    )
Select
    src.Date
,   Concat( Case
        When DatePart( Month, src.Date ) = 12 And DatePart( ISO_WEEK, src.Date ) = 1 Then DatePart( Year, src.Date ) + 1
        When DatePart( Month, src.Date ) = 1 And DatePart( ISO_WEEK, src.Date ) > 50 Then DatePart( Year, src.Date ) - 1
        Else DatePart( Year, src.Date )
    End, N'-W', Right( Concat( N'00', DatePart( ISO_WEEK, src.Date ) ), 2 ) ) As IsoYearWeek
From
    Dates src
;

A:

SELECT cast(DATEPART(YYYY, CreationDate) as varchar) + '-W' + Right('0'+cast(DATEPART(ISO_WEEK,CreationDate) as Varchar),2)

