TSQL, break period by Month only for given year - tsql

Question

I'm trying to break membership periods by month, but only for given year, in our example it 2022
Self inclusive code below works fine but it breaks all period, I only trying to process by month for 2022. Code below also holds definition for test data inside (#t temp table). I'm using #numbers table as a tally. I'm using SQL Server 2017
/*
SELECT TOP 100 ROW_NUMBER() OVER(ORDER BY (SELECT NULL))-1 AS n
INTO #numbers FROM master.dbo.spt_values --- Select top 100 * from #t numbers
----- data prep part
SELECT * INTO #t FROM ( -- DROP TABLE IF EXISTS #t
SELECT 100 ID, CAST('2022-03-02' AS DATE) eStart , CAST('2022-03-15' AS DATE) eEnd, '1 Same Month island' note , 100 Idx
UNION SELECT 200, '2022-01-11' , '2022-03-27', '2 Same Month Start 1', 2001
UNION SELECT 200, '2022-02-15' , '2022-5-15', '2 Same Month Start 2', 2002
UNION SELECT 300, '2021-05-08' , '2024-12-31', '3 Future eEnd ' ,3000
UNION SELECT 440, '2022-10-15' , '2023-10-10', '4 Future end 2' ,4400
) b -- SELECT * FROM #t
*/
select DISTINCT t.ID,
CASE WHEN YEAR(DATEADD(MONTH, DATEDIFF(MONTH, 0, t.eStart) + n, 0)) < 2022 THEN eStart
WHEN YEAR(DATEADD(MONTH, DATEDIFF(MONTH, 0, t.eStart) + n, 0)) > 2022 THEN '1/1/2023' -- first day after 2022
ELSE CASE WHEN n = 0 then t.eStart ELSE dateadd(month, datediff(month, 0, t.eStart) + n, 0) END
END eStart,
case when n = datediff(month, eStart, eEnd) then t.eEnd
else eomonth(dateadd(month, datediff(month, 0, t.eStart) + n, 0)) END eEnd
-- ,n, note ,eStart eStartOrig, eEnd eEndOrig
from #t t
join #numbers n on n.n <= datediff(month, eStart, eEnd)
order by t.ID, eStart

Answer 1

My suggested approach would be to build a "months" table for the desired range, join with your membership data, and keep pairs with overlapping date ranges. From there you can adjust the start and end dates to be the more restrictive between the actual mempership period and the selected month.
When comparing date ranges for overlap, a standard comparison is Start1 < End2 AND Start2 < End1 when dealing with exclusive end-dates or date/times. When dealing with inclusive end dates (as is the case here), the comparison becomes Start1 <= End2 AND Start2 <= End1
The resulting logic would then be something like:
-- Membership data #t as defined in original post
SELECT
A.MonthStart,
EOMONTH(A.MonthStart) AS MonthEnd
INTO #months
FROM (VALUES (1), (2), (3), (4), (5), (6), (7), (8), (9), (10), (11), (12)) N(N)
CROSS APPLY (SELECT DATEFROMPARTS(2022, N.N, 1) AS MonthStart) A
SELECT * from #months
SELECT
t.ID,
CASE WHEN m.MonthStart >= t.eStart THEN m.MonthStart ELSE t.eStart END AS eStart,
CASE WHEN m.MonthEnd <= t.eEnd THEN m.MonthEnd ELSE t.eEnd END AS eEnd,
t.Idx
FROM #t t
JOIN #months m -- Overlapping months (inclusive dates)
ON m.MonthStart <= t.eEnd
AND t.eStart <= m.MonthEnd
ORDER BY t.ID, t.Idx, eStart -- Added t.Idx to better match desired results
See this db<>fiddle for a demo.
If you want to include prior and following membership date ranges, you can include extra rows in the #months table such as:
1900-01-01 to 2021-12-31
2023-01-01 to 12-31-9999
or specific full-year ranges if that is what is wanted.

Answer 2

select DISTINCT t.ID,
CASE WHEN
YEAR(DATEADD(MONTH, DATEDIFF(MONTH, 0, t.eStart) + n, 0)) < 2022
THEN eStart
WHEN YEAR(DATEADD(MONTH, DATEDIFF(MONTH, 0, t.eStart) + n, 0)) > 2022
THEN '1/1/2023' -- first day after 2022
ELSE
CASE WHEN n = 0
then t.eStart
ELSE dateadd(month, datediff(month, 0, t.eStart) + n, 0)
END
END eStart,
CASE WHEN YEAR(
CASE WHEN n = DATEDIFF(MONTH, t.eStart, t.eEnd) THEN t.eEnd ELSE EOMONTH(DATEADD(MONTH, DATEDIFF(MONTH, 0, t.eStart) + n, 0)) END) < 2022
THEN CASE
WHEN t.eEnd < '1/1/2022'
THEN t.eEnd
ELSE DATEADD(d,-1,'1/1/2022')
END
WHEN YEAR(
CASE WHEN
n = DATEDIFF(MONTH, t.eStart, t.eEnd) THEN t.eEnd ELSE EOMONTH(DATEADD(MONTH, DATEDIFF(MONTH, 0, t.eStart) + n, 0))
END ) > 2022
THEN t.eEnd
ELSE
CASE WHEN
n = DATEDIFF(MONTH, t.eStart, t.eEnd)
THEN t.eEnd
ELSE EOMONTH(DATEADD(MONTH, DATEDIFF(MONTH, 0, t.eStart) + n, 0))
END
END eEnd
from #t t
join #numbers n on n.n <= datediff(month, eStart, eEnd)