In SQL how to calculate days in a year based on a start date and the number of days lapsed - sql

Question

What would be the SQL to calculate the number of days in each year if I had a start date and the number of days that have lapsed?
For example, the date (ymd) 2013-01-01 and the days lapsed is 1000.
I would like the result to look like this
2013 = 365
2014 = 365
2015 = 270
Can this be written as a function like datediff?
I have tried using a calendar table, but of course, linking to this just gives me 2013 = 1000
My calendar table looks like this.
DATE_ID | DATE | CALENDAR_YEAR | FINANCIAL_YEAR
-----------------------------------------------
20130101 | 2013-01-01 | 2013 | 2013/14
This is what i have tried.
SELECT
D.FISCAL_YEAR, SUM([DAYS]) AS NUMBER_OF_DAYS
FROM [dbo].[FACT] F
LEFT JOIN [dbo].[DIM_DATE] D ON D.DATE_ID = F.DATE_ID
GROUP BY
D.FISCAL_YEAR
The result for this is.
FISCAL_YEAR | NUMBER_OF_DAYS
----------------------------
2013/14 |2820
2014/15 |6635
2015/16 |2409

Answer 1

I would personally build a tally table to do this. Once you build that, you can easly get every date and count the number of days in each year:
DECLARE #YMD date = '20130101',
#Lapsed int = 1000;
--Build a Tally table
WITH N AS(
SELECT N
FROM (VALUES(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL)) N(N)),
Tally AS(
SELECT ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) -1 AS I
FROM N N1, N N2, N N3, N N4), --10,000 should be enough
--Build the dates table
Dates AS(
SELECT DATEADD(DAY, T.I, #YMD) AS CalendarDate
FROM Tally T
WHERE T.I <= #Lapsed - 1)
--And count the days
SELECT DATEPART(YEAR, CalendarDate) AS Year,
COUNT(CalendarDate) AS Days
FROM Dates D
GROUP BY DATEPART(YEAR, CalendarDate);
As a function:
CREATE FUNCTION CountDays (#YMD date, #Lapsed int)
RETURNS table
AS RETURN
--Build a Tally table
WITH N AS(
SELECT N
FROM (VALUES(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL)) N(N)),
Tally AS(
SELECT ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) -1 AS I
FROM N N1, N N2, N N3, N N4), --10,000 should be enough
--Build the dates table
Dates AS(
SELECT DATEADD(DAY, T.I, #YMD) AS CalendarDate
FROM Tally T
WHERE T.I <= #Lapsed - 1)
--And count the days
SELECT DATEPART(YEAR, CalendarDate) AS Year,
COUNT(CalendarDate) AS Days
FROM Dates D
GROUP BY DATEPART(YEAR, CalendarDate);
GO
SELECT *
FROM (VALUES('20130101',1000),
('20150501',755))V(YMD, Lapsed)
CROSS APPLY dbo.CountDays(V.YMD,V.Lapsed) CD;

Answer 2

One method is a recursive CTE:
with dates as (
select v.d, 1000 - datediff(day, v.d, dateadd(year, 1, v.d)) as days
from (values (datefromparts(2013, 1, 1))) v(d)
union all
select dateadd(year, 1, d), days - datediff(day, d, dateadd(year, 1, d))
from dates
where days > 0
)
select d,
(case when days > 0 then datediff(day, d, dateadd(year, 1, d))
else datediff(day, d, dateadd(year, 1, d)) + days
end)
from dates;
Here is a db<>fiddle.