Get Start and End Date from Week Number SQL Server

Question

We have WeekNo and Year entered into a table in following format:

YYYY-ww     (2021-48)

I want start date and end date from given week no with year as above format.

How to get it from SQL query or procedure?

Thanks,

Answer 1

How about this?

declare @dateText varchar(7);
declare @week int;
declare @year int;

set datefirst 1;              -- Set first day of week to Monday
set @dateText = '2021-48';
set @year = SUBSTRING(@dateText, 1, 4)
set @week = SUBSTRING(@dateText, 6, 2)

select DATEADD(week, @week, DATEADD(year, @year-1900, 0)) - 4 -
     DATEPART(dw, DATEADD(week, @week, DATEADD(year, @year-1900, 0)) - 4) + 1

Output
-----------------
2021-11-29 00:00:00.000

Alternatively you could create a function:

CREATE FUNCTION dbo.GetStartOfWeek(@dateText varchar(7))
RETURNS Datetime
BEGIN
    
declare @week int;
declare @year int;
declare @offset int;

select
    @offset =  @@DATEFIRST - 1, 
    @year = SUBSTRING(@dateText, 1, 4),
    @week = SUBSTRING(@dateText, 6, 2)

 RETURN DATEADD(week, @week, DATEADD(year, @year-1900, 0)) - 4 -
        DATEPART(dw, DATEADD(week, @week, DATEADD(year, @year-1900, 0) + @offset) - 4) + 1
END;

Which you can use in a SQL query or stored procedure:

select dbo.GetStartOfWeek('2021-48') as StartDate, DATEADD(d, 7, dbo.GetStartOfWeek('2021-48')) as EndDate

Answer 2

(An extra answer, since this method is a bit different.)

This method is about using a tally table with dates and date related stuff.

Then such calendar table can be used to lookup the year and week.

For example

Create calendar table

create table REF_Calendar (
 cal_date date primary key not null,
 cal_year smallint not null,
 cal_month tinyint not null,
 cal_day tinyint not null,
 cal_dayofyear smallint not null,
 cal_quarter tinyint not null,
 cal_weekday_abbrev char(3) not null,
 cal_iso_week tinyint,
 cal_df7_week tinyint not null,
 cal_df7_weekday tinyint not null,
 cal_df1_week tinyint,
 cal_df1_weekday tinyint,
 cal_mooncycle tinyint,
 index idx_Calendar_year_week unique (cal_year, cal_df7_week, cal_df7_weekday)
);

Fill the calendar

SET DATEFIRST 7; -- 7: week starts on sunday

declare @startdate date = '2020-01-01';
declare @enddate   date = '2023-01-01';

with rcte_calendar as (
  select @startdate as cal_date
  union all
  select dateadd(day, 1, cal_date)
  from rcte_calendar
  where cal_date < @enddate
)
insert into ref_calendar 
(cal_date, cal_year, cal_month, cal_day, cal_dayofyear, cal_quarter, cal_weekday_abbrev, 
 cal_df7_week, cal_df7_weekday, cal_mooncycle)
select
  cal_date
, datepart(year, cal_date) as cal_year
, datepart(month, cal_date) as cal_month
, datepart(day, cal_date) as cal_day
, datepart(dayofyear, cal_date) as cal_dayofyear
, datepart(quarter, cal_date) as cal_quarter
, lower(left(datename(weekday, cal_date), 3)) as cal_weekday_abbrev
, datepart(week, cal_date) as cal_df7_week
, datepart(weekday, cal_date) as cal_df7_weekday
, abs(1-abs(round(((14-1.0*CONVERT(int, CONVERT(nvarchar(2), cal_date, 131)))/14)
 , 1, 1)))*100 as cal_mooncycle
from rcte_calendar cte
where not exists (
        select 1 
        from ref_calendar cal
        where cal.cal_date = cte.cal_date
      )
option (maxrecursion 0);

Updating extra's

SET DATEFIRST 1; -- 1: week starts on monday
update ref_calendar
set cal_df1_week = datepart(week, cal_date)
  , cal_df1_weekday = datepart(weekday, cal_date)
  , cal_iso_week = datepart(iso_week, cal_date)
where cal_df1_week is null
   or cal_df1_weekday is null
   or cal_iso_week is null;

Check how it looks

select top 9 * 
from ref_calendar 
where cal_mooncycle = 100
  and cal_date >= getdate()
  and cal_year <= 1 + datepart(year, current_timestamp)
order by cal_date asc;

cal_date	cal_year	cal_month	cal_day	cal_dayofyear	cal_quarter	cal_weekday_abbrev	cal_iso_week	cal_df7_week	cal_df7_weekday	cal_df1_week	cal_df1_weekday	cal_mooncycle
2021-12-17	2021	12	17	351	4	fri	50	51	6	51	5	100
2021-12-18	2021	12	18	352	4	sat	50	51	7	51	6	100
2021-12-19	2021	12	19	353	4	sun	50	52	1	51	7	100
2022-01-16	2022	1	16	16	1	sun	2	4	1	3	7	100
2022-01-17	2022	1	17	17	1	mon	3	4	2	4	1	100
2022-01-18	2022	1	18	18	1	tue	3	4	3	4	2	100
2022-02-14	2022	2	14	45	1	mon	7	8	2	8	1	100
2022-02-15	2022	2	15	46	1	tue	7	8	3	8	2	100
2022-02-16	2022	2	16	47	1	wed	7	8	4	8	3	100

Test applying the calendar to a table

DECLARE @Test TABLE (col char(7)); 
INSERT INTO @Test VALUES 
  ('2021-01'), ('2021-02')
, ('2021-48')
, ('2022-01'), ('2022-02')
;

select *
from @test t
outer apply (
  select 
    min(cal_date) as startofweek
  , max(cal_date) as endofweek
  from (values (try_cast(left(col, 4) as int),
            abs(try_cast(right(col, 2) as int))) 
  ) as val(yr, ww)
  join ref_calendar as cal
    on ((cal_year = yr and cal_df7_week = ww)
        or (ww = 1  and cal_year = yr-1 and cal_df7_week = 53) 
        or (ww = 53 and cal_year = yr+1 and cal_df7_week = 1))
) cal

col	startofweek	endofweek
2021-01	2020-12-27	2021-01-02
2021-02	2021-01-03	2021-01-09
2021-48	2021-11-21	2021-11-27
2022-01	2021-12-26	2022-01-01
2022-02	2022-01-02	2022-01-08

Demo on db<>fiddle here

Answer 3

Here are 2 ways to calculate the start & end day of a Year-Week string in a query.

1) Without UDF

SET DATEFIRST 7; -- 7: Sunday has weekday 1
DECLARE @Test TABLE ([YEAR-WW] char(7)); 
INSERT INTO @Test VALUES 
  ('2020-53'), ('2021-01'), ('2021-02')
, ('2021-48')
, ('2021-53'), ('2022-01');

SELECT [YEAR-WW]
, [FirstDayOfWeek] = CAST(DATEADD(day, 1-DATEPART(weekday, DATEADD(week, ABS(RIGHT([YEAR-WW],2))-1, LEFT([YEAR-WW],5)+'01-01')), DATEADD(week, ABS(RIGHT([YEAR-WW],2))-1, LEFT([YEAR-WW],5)+'01-01')) AS DATE)
, [LastDayOfWeek]  = CAST(DATEADD(day, 6, DATEADD(day, 1-DATEPART(weekday, DATEADD(week, ABS(RIGHT([YEAR-WW],2))-1, LEFT([YEAR-WW],5)+'01-01')), DATEADD(week, ABS(RIGHT([YEAR-WW],2))-1, LEFT([YEAR-WW],5)+'01-01'))) AS DATE)
FROM @Test
ORDER BY 1;

YEAR-WW	FirstDayOfWeek	LastDayOfWeek
2020-53	2020-12-27	2021-01-02
2021-01	2020-12-27	2021-01-02
2021-02	2021-01-03	2021-01-09
2021-48	2021-11-21	2021-11-27
2021-53	2021-12-26	2022-01-01
2022-01	2021-12-26	2022-01-01

2) Using this UDF

CREATE FUNCTION dbo.GetDateFromYearWeek (
 @YearWeek VARCHAR(7) = '000101', -- default
 @WeekDay INT = 1, -- default
 @FirstWeekDayName VARCHAR(9) = 'mon' -- default
) RETURNS DATE
BEGIN
 
 IF @YearWeek = '000101'
   SET @YearWeek = CONCAT(DATEPART(year, GETDATE()), '-', DATEPART(week, GETDATE())); 
 
 IF @YearWeek NOT LIKE '[0-9][0-9][0-9][0-9]%[0-9-][0-9]'
   RETURN NULL;
 
 IF @WeekDay < 1 OR @WeekDay > 7
   RETURN NULL;
 
 DECLARE @FirstWeekDay INT = CHARINDEX(LOWER(LEFT(@FirstWeekDayName,3)), '   montuewedthufrisatsun')/3;
 IF @FirstWeekDay = 0 -- not found in string
   SET @FirstWeekDay = @@DATEFIRST;
 
 DECLARE @Year INT = TRY_CAST(LEFT(@YearWeek, 4) AS INT);
 DECLARE @Week INT = ABS(TRY_CAST(RIGHT(@YearWeek, 2) AS INT));
 
 DECLARE @Date DATE = TRY_CAST(CONCAT(@Year,'-01-01') AS DATE);
 SET @Date = DATEADD(week, @Week-1, @Date);
 
 DECLARE @DowDiff INT = (6-@FirstWeekday+@@DATEFIRST+DATEPART(weekday,@Date))%7;
 SET @Date = DATEADD(day, -@DowDiff, @Date);
 SET @Date = DATEADD(day, @WeekDay-1, @Date);

 RETURN @Date;
END;

Example usage

DECLARE @Test TABLE ([YEAR-WW] char(7)); 
INSERT INTO @Test VALUES 
  ('2020-53'), ('2021-01'), ('2021-02')
, ('2021-48')
, ('2021-53'), ('2022-01'), ('2022-02')
;

SELECT [YEAR-WW]
, [FirstOfWeek_Mon] = dbo.GetDateFromYearWeek([YEAR-WW], 1, 'mon')
, [LastOfWeek_Mon]  = dbo.GetDateFromYearWeek([YEAR-WW], 7, 'Monday')
, [FirstOfWeek_Sun] = dbo.GetDateFromYearWeek([YEAR-WW], 1, 'sun')
, [LastOfWeek_Sun]  = dbo.GetDateFromYearWeek([YEAR-WW], 7, 'SUNDAY')
FROM @Test
ORDER BY 1;

YEAR-WW	FirstOfWeek_Mon	LastOfWeek_Mon	FirstOfWeek_Sun	LastOfWeek_Sun
2020-53	2020-12-28	2021-01-03	2020-12-27	2021-01-02
2021-01	2020-12-28	2021-01-03	2020-12-27	2021-01-02
2021-02	2021-01-04	2021-01-10	2021-01-03	2021-01-09
2021-48	2021-11-22	2021-11-28	2021-11-21	2021-11-27
2021-53	2021-12-27	2022-01-02	2021-12-26	2022-01-01
2022-01	2021-12-27	2022-01-02	2021-12-26	2022-01-01
2022-02	2022-01-03	2022-01-09	2022-01-02	2022-01-08

Demo on db<>fiddle here