Calculation of month between 2 dates by year - sql

Question

Need help in understanding on how to Calculate the months by year between 2 dates , able to calculate months between the dates but not by year
ID StartDate Enddate
1 1/1/2016 4/23/2019
2 1/1/2016 4/30/2017
3 1/1/2016 12/31/2018
4 1/1/2017 4/23/2019
5 5/20/2017 11/30/2017
ID StartDate Enddate 2016 2017 2018 2019
1 1/1/2016 4/23/2019 12 12 12 4
2 1/1/2016 4/30/2017 12 4 0 0
3 1/1/2016 12/31/2018 12 12 12 0
4 1/1/2017 4/23/2019 0 12 12 4
5 5/20/2017 11/30/2017 0 7 0 0

Answer 1

Counting intervals like this is tricky in SQL Server. It would be much simpler if the database supported least() and greatest(). Here is a verbose approach:
select id, startdate, enddate,
(case when startdate >= '2017-01-01' or enddate < '2016-01-01' then 0
when startdate < '2016-01-01' and enddate >= '2017-01-01' then 12
when startdate >= '2016-01-01' and enddate >= '2017-01-01' then 13 - month(startdate)
when startdate < '2016-01-01' then month(enddate)
else month(enddate) + 1 - month(startdate)
end) as months_2016,
(case when startdate >= '2018-01-01' or enddate < '2017-01-01' then 0
when startdate < '2017-01-01' and enddate >= '2018-01-01' then 12
when startdate >= '2017-01-01' and enddate >= '2018-01-01' then 13 - month(startdate)
when startdate < '2017-01-01' then month(enddate)
else month(enddate) + 1 - month(startdate)
end) as months_2017,
(case when startdate >= '2019-01-01' or enddate < '2018-01-01' then 0
when startdate < '2018-01-01' and enddate >= '2019-01-01' then 12
when startdate >= '2018-01-01' and enddate >= '2019-01-01' then 13 - month(startdate)
when startdate < '2018-01-01' then month(enddate)
else month(enddate) + 1 - month(startdate)
end) as months_2018
from t;
This basically handles the different conditions on whether the start date is before, during, or after the year in question and the same for the end date.

Answer 2

If you have the years in a table you can do a dynamic pivot like this, it would save you declaring the columns by hand:
CREATE TABLE #Data (ID int, StartDate date, EndDate date);
CREATE TABLE #Year (y int);
SET DATEFORMAT MDY;
insert into #Data
values
(1, '1/1/2016','4/23/2019'),
(2, '1/1/2016','4/30/2017'),
(3, '1/1/2016','12/31/2018'),
(4, '1/1/2017','4/23/2019'),
(5, '5/20/2017','11/30/2017')
;
insert into #Year
values
(2016),
(2017),
(2018),
(2019)
;
DECLARE
#PivotColumnNames AS NVARCHAR(MAX),
#DynamicPivotQuery AS NVARCHAR(MAX);
SELECT
#PivotColumnNames = ISNULL(#PivotColumnNames + ',','') + QUOTENAME(Y)
FROM
#Year;
SELECT
#DynamicPivotQuery =
'SELECT
ID,
StartDate,
EndDate,'
+ #PivotColumnNames +
'FROM
(
select
ID,
StartDate,
EndDate,
Y,
case
when StartDate >= DATEFROMPARTS(Y+1, 1, 1) or EndDate < DATEFROMPARTS(Y, 1, 1) then 0
when StartDate < DATEFROMPARTS(Y, 1, 1) and EndDate >= DATEFROMPARTS(Y+1, 1, 1) then 12
when StartDate >= DATEFROMPARTS(Y, 1, 1) and EndDate >= DATEFROMPARTS(Y+1, 1, 1) then 13 - MONTH(StartDate)
when StartDate < DATEFROMPARTS(Y, 1, 1) then month(EndDate)
else MONTH(EndDate) + 1 - MONTH(StartDate)
end M
from
#Data, #Year
) as SourceTable
PIVOT
(
SUM(M)
FOR Y IN ('
+ #PivotColumnNames +
')
) as PivotTable';
SELECT #DynamicPivotQuery;
EXEC sp_executesql #DynamicPivotQuery;
DROP TABLE #Data, #Year;