Counting the # of days in each month, subsequent of a date - sql

Question

I'm trying to see if I can calculate the # of days in each month, after a given date for a certain # of days.
For example, I have a date of 2019-09-25. If I am planning for the next 105 days, how many of those days are in September, October, November, and so on?
Declare #dtdate date = '20190925',
#days int= 105
Select
datediff(dd,#dtdate,eomonth(#dtdate)) as DaysSeptember
,datediff(dd,eomonth(#dtdate),eomonth(dateadd(m,1,#dtdate))) as DaysOctober

Answer 1

It looks to me like Sql server. You can do it by simply counting days in each month. Doing it this way has an advantage of flexibility. You can simply change #dtdate, #days and the query will work despite of changing number of months.
DECLARE #dtdate date = '20190925',
#days int= 105
,#dtmax date;
set #dtmax = dateadd(day, #days, #dtdate);
WITH cte AS (
SELECT DATEADD(DAY, ROW_NUMBER() OVER(ORDER BY 1/0), #dtdate) AS d
FROM sys.objects s, sys.objects s2
)
select
year(d) as year, datename(month, d) as month, count(*) as NumberOfDays
from cte
where d between #dtdate and #dtmax
group by year(d), datename(month, d)
order by year(d), month
result:
year month NumberOfDays
2019 December 31
2019 November 30
2019 October 31
2019 September 5
2020 January 8

Answer 2

In postgresql I'd do this by
SELECT date_part('month', d), count(d)
FROM generate_series('2019-09-25'::date, '2019-09-25'::date + INTERVAL '105 days', INTERVAL '1 day') series (d)
GROUP BY date_part('year',d), date_part('month', d)
ORDER BY date_part('year',d), date_part('month', d)
Clearly you're not using postgresql but perhaps this will give you a hint. The trick is to create a series of dates within the interval that you can then count. Here'st the output with the month and number of days. Note there are 106 days because the interval is inclusive of the beginning and end dates.
9;6
10;31
11;30
12;31
1;8

Answer 3

Perhaps not the best solution (not set based), but an alternative approach:
DECLARE #dtdate DATE = '20190925'
,#days INT = 105
,#DaysInMonth INT
DECLARE #results TABLE
(
ResultsID INT NOT NULL IDENTITY PRIMARY KEY
,YearMonth VARCHAR(7) NOT NULL
,DaysInMonth INT NOT NULL
)
WHILE #days > 0
BEGIN
SET #DaysInMonth = DATEDIFF(dd, #dtdate, EOMONTH(#dtdate))
SET #DaysInMonth = IIF(#days > #DaysInMonth, #DaysInMonth, #days)
INSERT INTO #results
(
YearMonth
,DaysInMonth
)
SELECT CONVERT(VARCHAR(7), #dtdate, 120)
,#DaysInMonth
SET #days -= #DaysInMonth
SET #dtdate = DATEADD(dd, 1, EOMONTH(#dtdate))
END
SELECT *
FROM #results AS r