Calculate calendar days by month for date ranges - sql

Question

I have a table with date ranges:
CREATE TABLE REQUEST (
REQUEST_ID NUMBER(*,0) NOT NULL ENABLE,
EMPLOYEE_ID NUMBER(*,0) NOT NULL ENABLE,
START_DATE DATE NOT NULL ENABLE,
END_DATE DATE NOT NULL ENABLE,
CONSTRAINT REQUEST_PK PRIMARY KEY (REQUEST_ID)
);
The're also a couple of constraints (omitted for brevity) that ensure they're valid (end date cannot be less then start date) and force time to be 00:00:00. (Runnable fiddle with sample data).
Is there a way to split my data set by year/month into a result set based on the date ranges? The rules are:
A request splits in as many rows as months its range covers.
Each month has a column stating how many calendar days the range has in such month.
For example, a request with [2020-12-28, 2021-02-10] would produce three rows:
request_id year month days
========== ==== ===== ====
1 2020 12 4
1 2021 1 31
1 2021 2 10
I've been playing with CONNECT BY but I've been unable to adapt it to my use case. Is that the right tool?

Answer 1

A more direct way to do the computation is to use connect by to generate just the needed months (not every day in every interval) - and then to do the day computation directly, rather than by counting. Something like this:
Adding data for testing:
insert into request (request_id, employee_id, start_date, end_date)
select 1, 1001, date '2020-12-28', date '2021-02-10' from dual union all
select 2, 4002, date '2021-02-10', date '2021-02-20' from dual union all
select 3, 6004, date '2020-12-15', date '2021-03-31' from dual
;
commit;
Query and output:
with
prep (request_id, start_date, end_date, mth) as (
select request_id, start_date, end_date,
add_months(trunc(start_date, 'mm'), level - 1)
from request
connect by level <= months_between(trunc(end_date, 'mm'),
trunc(start_date, 'mm')) + 1
and prior request_id = request_id
and prior sys_guid() is not null
)
select request_id, extract(year from mth) as year_,
extract(month from mth) as month_,
least(last_day(mth), end_date) - greatest(mth, start_date) + 1 as days
from prep
order by request_id, mth -- if needed
;
REQUEST_ID YEAR_ MONTH_ DAYS
---------- ---------- ---------- ----------
1 2020 12 4
1 2021 1 31
1 2021 2 10
2 2021 2 11
3 2020 12 17
3 2021 1 31
3 2021 2 28
3 2021 3 31

Answer 2

For example this one:
WITH t AS (
SELECT DATE '2020-12-28' +LEVEL-1 AS ts
FROM dual
CONNECT BY DATE '2020-12-28' +LEVEL-1 <= DATE '2021-02-10')
SELECT
EXTRACT(YEAR FROM TRUNC(ts, 'Month')) AS YEAR,
EXTRACT(MONTH FROM TRUNC(ts, 'Month')) AS MONTH,
COUNT(ts) AS DAYS
FROM t
GROUP BY TRUNC(ts, 'Month')
ORDER BY YEAR, MONTH;