从周数 SQL 服务器获取开始和结束日期

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;
select @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)) - 4) + 1
END;

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

select dbo.GetStartOfWeek('2021-48')

Answer 2

Here's a test snippet to calculate the start & end day of a Year-Week in a query. Without using a User-Defined-Function.

SET datefirst 1;
DECLARE @Test TABLE ([YEAR-WW] char(7)); 
INSERT INTO @Test VALUES 
('2021-01'), ('2021-02'), 
('2021-48'), 
('2021-53'), 
('2022-01'), ('2022-02');

SELECT 
  [YEAR-WW]
, [StartOfWeek] = CAST(DATEADD(day, 1-DATEPART(weekday, DATEADD(week, ABS(RIGHT([YEAR-WW],2))-2, LEFT([YEAR-WW],5)+'01-07')), DATEADD(week, ABS(RIGHT([YEAR-WW],2))-2, LEFT([YEAR-WW],5)+'01-07')) AS DATE)
, [EndOfWeek]   = CAST(DATEADD(day, 6, DATEADD(day, 1-DATEPART(weekday, DATEADD(week, ABS(RIGHT([YEAR-WW],2))-2, LEFT([YEAR-WW],5)+'01-07')), DATEADD(week, ABS(RIGHT([YEAR-WW],2))-2, LEFT([YEAR-WW],5)+'01-07'))) AS DATE)
FROM @Test
ORDER BY 1;

 YEAR-WW | StartOfWeek | EndOfWeek:------ |:---------- |:--------- 2021-01 |  2020-12-28 |  2021-01-03 2021-02 |  2021-01-04 |  2021-01-10 2021-48 |  2021-11-22 |  2021-11-28 2021-53 |  2021-12-27 |  2022-01-02 2022-01 |  2021-12-27 |  2022-01-02 2022-02 |  2022-01-03 |  2022-01-09 

Demo on db<>fiddle here

Answer 3

(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 table REF_Calendar ( cal_date date primary key not null, cal_year smallint not null, cal_dayofyear smallint not null, cal_quarter tinyint not null, cal_weekday_abbrev char(3) not null, cal_iso_week tinyint not null, cal_df7_week tinyint not null, cal_df7_weekday tinyint not null, 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_dayofyear, cal_quarter, cal_weekday_abbrev, cal_iso_week, cal_df7_week, cal_df7_weekday, cal_mooncycle) select cal_date, datepart(year, cal_date) as cal_year, 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(iso_week, cal_date) as cal_iso_week, 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);

 select top 2 * from ref_calendar order by cal_date desc

cal_date	cal_year	cal_dayofyear	cal_quarter	cal_weekday_abbrev	cal_iso_week	cal_df7_week	cal_df7_weekday	cal_mooncycle
2023-01-01	2023	1	1	sun	52	1	1	70
2022-12-31	2022	365	4	sat	52	53	7	60

Test

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(c.cal_date) as startofweek, max(c.cal_date) as endofweek from ref_calendar c cross join (values (try_cast(left(col, 4) as int), abs(try_cast(right(col, 2) as int))) ) as q(yr, ww) where (c.cal_year = yr and c.cal_df7_week = ww) or (ww = 1 and c.cal_year = yr-1 and c.cal_df7_week = 53 and c.cal_dayofyear > 349) or (ww = 53 and c.cal_year = yr+1 and c.cal_df7_week = 1 and c.cal_dayofyear < 7) ) 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