从周和年获取第一个和最后一个日期

Question

I am trying to retrieve the first and last date from a week and year in T-SQL. I found some exemples in stackoverflow or in google but nothing work well. The better exemple I find is this one :

DECLARE @WeekNo int= 20
DECLARE @Year int=2019

SELECT DATEADD(wk,@WeekNo-1,DATEADD(yy,@Year-1900,0)) AS WeekStart,
DATEADD(wk,@WeekNo,DATEADD(yy,@Year-1900,0))-1 AS WeekEnd

But I need the week start on a monday. And if I change it, it works for 2019 and not 2020. I am really stuck on this point.

Edit : First day of the week need to be monday. And week number are iso formated. First week of the year start only if 1 january is before a Friday (4 days in nexw year needed to be first week)

Answer 1

You can try this below logic to find start and end date of a week from week and year number. This will consider Monday as a week starter.

(Logic taken from online search)

DECLARE @year INT, @isoweek INT

SET @year = 2020
SET @isoweek = 14

SELECT
DATEADD(
    d, 
    (@isoweek - 1) * 7,
    CASE (DATEPART(dw, CAST(@year AS CHAR(4)) + '-01-04') + @@DATEFIRST - 1) % 7
        WHEN 1 THEN CAST(@year AS CHAR(4)) + '-01-04'
        WHEN 2 THEN DATEADD(d, -1, CAST(@year AS CHAR(4)) + '-01-04')
        WHEN 3 THEN DATEADD(d, -2, CAST(@year AS CHAR(4)) + '-01-04')
        WHEN 4 THEN DATEADD(d, -3, CAST(@year AS CHAR(4)) + '-01-04')
        WHEN 5 THEN DATEADD(d, -4, CAST(@year AS CHAR(4)) + '-01-04')
        WHEN 6 THEN DATEADD(d, -5, CAST(@year AS CHAR(4)) + '-01-04')
        ELSE DATEADD(d, -6, CAST(@year AS CHAR(4)) + '-01-04')
    END

) AS startdate , 

DATEADD(
    d, 
    (@isoweek) * 7 - 1,
    CASE (DATEPART(dw, CAST(@year AS CHAR(4)) + '-01-04') + @@DATEFIRST - 1) % 7
        WHEN 1 THEN CAST(@year AS CHAR(4)) + '-01-04'
        WHEN 2 THEN DATEADD(d, -1, CAST(@year AS CHAR(4)) + '-01-04')
        WHEN 3 THEN DATEADD(d, -2, CAST(@year AS CHAR(4)) + '-01-04')
        WHEN 4 THEN DATEADD(d, -3, CAST(@year AS CHAR(4)) + '-01-04')
        WHEN 5 THEN DATEADD(d, -4, CAST(@year AS CHAR(4)) + '-01-04')
        WHEN 6 THEN DATEADD(d, -5, CAST(@year AS CHAR(4)) + '-01-04')
        ELSE DATEADD(d, -6, CAST(@year AS CHAR(4)) + '-01-04')
    END
) AS enddate

Answer 2

I would suggest using a reference monday (in the past), which is your base in order to find the monday in your week. Following an example:

DECLARE @WeekNo int= 20
DECLARE @Year int=2020
DECLARE @RefMonday date = CAST('1990-01-01' AS DATE)

DECLARE @d date = DATEADD(d, 7*@WeekNo, CAST(CAST(@Year AS VARCHAR(4))+'-01-01' AS DATE))
DECLARE @offset int = DATEDIFF(d, @RefMonday, @d)%7

DECLARE @StartOfWeek date = (SELECT DATEADD(DD, -1 * @offset, @d))
DECLARE @EndOfWeek date = DATEADD(d, 6, @StartOfWeek)

SELECT @StartOfWeek AS StartOfWeek, @EndOfWeek AS EndOfWeek