Auto pickup dates from SQL Server - T-SQL - sql

Question

I am looking for some T-SQL code that should pick the date which is "One Year back from current date (at the same time last Sunday in the month of January)".
For example:
Current day expected result
2017-02-05 2016-01-31
2017-01-05 2015-01-25
2018-02-19 2017-01-29
2018-01-19 2016-01-31
2019-02-28 2018-01-28
Please note: The year starts from last Sunday in January
I have some T-SQL code which is being used in SQL Server 2014:
select
convert(varchar(10), DATEADD(day, DATEDIFF(day, '19000107', DATEADD(month, DATEDIFF(MONTH, 0, CONVERT(date, CONVERT(VARCHAR(4), (CASE WHEN MONTH(GetDate()) = 1 THEN CONVERT(VARCHAR(4), GetDate(), 112) - 1 ELSE CONVERT(VARCHAR(4), GetDate(), 112) END), 112) + '0101')), 30)) / 7 * 7, '19000107'), 120)
The above code picks the date for current year's (last Sunday in January month). But I want T-SQL code to pick last year's (last Sunday's date in January month) date.
In detail - I want T-SQL code to produce expected result from below table
Current day T-SQL code answer expected result
2017-02-05 2017-01-29 2016-01-31
2017-01-05 2016-01-31 2015-01-25
2018-02-19 2018-01-28 2017-01-29
2018-01-19 2017-01-29 2016-01-31
2019-02-28 2019-01-27 2018-01-28
Any help please.

Answer 1

The best thing for this question is a numbers and date table. This answer shows you how to create one. Such a table is very handsome in many situations...
If I understand this correctly, you want the last Sunday in January of the previous year in all cases? Try this:
DECLARE #dummy TABLE(ID INT IDENTITY,YourDate DATE);
INSERT INTO #dummy VALUES
('2017-02-05'),('2017-01-05'),('2018-02-19'),('2018-01-19'),('2019-02-28');
WITH Years AS
(
SELECT * FROM (VALUES(2010),(2011),(2012),(2013),(2014),(2015),(2016),(2017),(2018),(2019),(2020)) AS t(Yr)
)
,LastSundays AS
(
SELECT Yr AS TheYear
,DATEADD(DAY,(DATEPART(WEEKDAY,LastOfJanuary) % 7)*(-1),LastOfJanuary) AS LastSundayOfJanuary
FROM Years
CROSS APPLY(SELECT CAST(CAST(Yr AS VARCHAR(4)) + '0131' AS DATE)) AS t(LastOfJanuary)
)
SELECT *
FROM #dummy AS d
INNER JOIN LastSundays AS ls ON YEAR(DATEADD(YEAR,-1,d.YourDate))=ls.TheYear;
The result (I do not understand row 2 and 4 completely...)
ID YourDate TheYear LastSundayOfJanuary
1 2017-02-05 2016 2016-01-31
2 2017-01-05 2016 2016-01-31 <--Your sample data is different...
3 2018-02-19 2017 2017-01-29
4 2018-01-19 2017 2017-01-29 <--Your sample data is different...
5 2019-02-28 2018 2018-01-28
Hint You might need to introduce ##DATEFIRST into your calculations...

Answer 2

Here is a way to do it without a date table (which is still a good idea BTW). Tested on all your inputs and it delivers the correct output each time. Obviously you would refactor this a bit as it's longwinded, just to show each step.
/* The input date. */
DECLARE
#input DATE = '2019-02-28';
/* The input date less one year. */
DECLARE
#date_minus_one_year DATE = DATEADD(yy,-1,#input);
/* The year part of the input date less one year. */
DECLARE
#year_date_part INT = DATEPART(yy,#date_minus_one_year);
/* 31 Jan of the previous year. */
DECLARE
#prev_year_jan_eom DATE = CAST(CAST(#year_date_part AS VARCHAR(4))+'-01-31' AS DATE);
/* What day of the week is 31 Jan of the previous year? */
DECLARE
#previous_eom_dw_part INT = DATEPART(dw,#prev_year_jan_eom);
/* Offest 31 Jan to the previous Sunday, won't change if the 31st is itself a Sunday. */
DECLARE
#output DATE = DATEADD(dd,1 - #previous_eom_dw_part,#prev_year_jan_eom);
/* Input and output */
SELECT
#input input
,#output [output];

Answer 3

I didn't think of a way to do it without the conditional in a case. It also uses the trick of casting a numeric year value to a January 1st date.
select case
when
datepart(dayofyear, dt) >
31 - datepart(weekday, dateadd(day, 30, cast(year(dt) as varchar(4))))
then
dateadd(day,
31 - datepart(weekday, dateadd(day, 30, cast(year(dt) as varchar(4)))),
cast(year(dt) as varchar(4))
)
else
dateadd(day,
31 - datepart(weekday, dateadd(day, 30, cast(year(dt) - 1 as varchar(4)))),
cast(year(dt) - 1 as varchar(4))
)
end
from (values
('20100201'), ('20110301'), ('20120401'),
('20130501'), ('20140601'), ('20150701'),
('20160801'), ('20170901'), ('20181001')
) t(dt)
Just for fun (untested)
select
dateadd(week,
-52 * ceil(sign(datediff(day, dt, hs)) + 0.5),
js
)
from
(select <date> dt) as t
cross apply
(
select 31 - datepart(weekday,
datefromparts(year(dt), 1, 31) as js
) t2;

Answer 4

SELECT
convert(varchar(10), DATEADD(day, DATEDIFF(day, '19000107', DATEADD(month, DATEDIFF(MONTH, 0, CONVERT(date, CONVERT(VARCHAR(4), (CASE WHEN MONTH(DATEADD(year,-1,GetDate())) = 1 THEN CONVERT(VARCHAR(4), DATEADD(year,-1,GetDate()), 112) - 1 ELSE CONVERT(VARCHAR(4), DATEADD(year,-1,GetDate()), 112) END), 112) + '0101')), 30)) / 7 * 7, '19000107'), 120)