SQL Server : create summarization based on multiple dates - sql

Question

I have the following table containing positions for workers dated back by 10 years:
worker_id
position_code
date_from
date_to
1
x1
2021-01-01
2100-12-31
1
x2
2020-12-01
2021-01-01
2
x3
2000-01-01
2100-12-31
I want to create a view, where I can see for each worker what their position for every month.
So for example:
year
month
worker_id
position_code
2020
12
1
x2
2020
12
2
x3
2021
1
1
x1
2021
1
2
x3
2021
2
1
x1
Ideally I'm only interested on the last 6 month to have better performance.
overall there is ~10000 workers, and the table itself around ~100000 lines.
for some workers there is only 1 position, but it can be multiple.
In theory position is only changing at the beginning of months, but would be better to watch for this as well, and in this case take the which is active at the end of the month.
(so for example: from jan 1-10 position is x1, from jan 10-to 31 x2, in this case x2 is the one I'm looking for)

Answer 1

WITH WORKERS(worker_id, position_code, date_from, date_to) AS
(
SELECT 1 , 'x1', '2021-01-01', '2100-12-31' UNION ALL
SELECT 1 , 'x2' , '2020-12-01', '2021-01-01' UNION ALL
SELECT 2 , 'x3' , '2000-01-01' , '2100-12-31'
),
MINI_MAX AS
(
SELECT MIN(DATE_FROM)AS STARTT_DATE,MAX(DATE_TO)AS END_DATE
FROM WORKERS
),
CALENDAR AS
(
SELECT CAST(STARTT_DATE AS DATE)DATE_D FROM MINI_MAX AS W
UNION ALL
SELECT DATEADD(MONTH,1,Z.DATE_D)
FROM CALENDAR AS Z
WHERE Z.DATE_D<=(SELECT END_DATE FROM MINI_MAX)
),
RESULT AS
(
SELECT YEAR(C.DATE_D)AS YEARR,MONTH(C.DATE_D)MONTHH,W.worker_id,W.position_code
FROM CALENDAR AS C
JOIN WORKERS AS W ON C.DATE_D BETWEEN W.date_from AND W.date_to
)
SELECT R.YEARR,R.MONTHH,R.worker_id,R.position_code
FROM RESULT AS R
OPTION(MAXRECURSION 0)
I would say that the most suitable way for this kind of queries is to use a permanent calendar table and perform JOIN directly to it

Answer 2

The hard part is generating the months. One method is a recursive CTE:
with cte as (
select worker_id, position_code, date_from as dte,
eomonth(case when date_to < eomonth(getdate()) then dateadd(day, -1, date_to) else getdate() end) as date_to
from t
union all
select worker_id, position_code,
dateadd(month, 1, datefromparts(year(dte), month(dte), 1)), date_to
from cte
where eomonth(dte) < eomonth(date_to)
)
select *
from cte
order by worker_id, dte desc
option (maxrecursion 0)
Note: You might get duplicates if a worker starts a position in the middle of a month.
Here is a db<>fiddle.