unstack start and end date to month - sql

Question

cust_id start end subs_price_p_month
1 2019-01-01 2019-12-10 50.00
1 2020-02-03 2020-08-05 39.99
2 2019-12-11 2020-11-08 29.99
I would like to "unstack" the table above, so that each row contains the subs price for 1 month:
cust_id month subs_price_p_month
1 2019-01-01 50.00
1 2019-02-01 50.00
1 2019-03-01 50.00
....
1 2019-12-01 50.00
1 2020-02-01 39.99
1 2020-03-01 39.99
1 2020-04-01 39.99
....
1 2020-08-01 39.99
2 2019-12-01 29.99
2 2020-01-01 29.99
2 2020-02-01 29.99
...
2 2020-11-01 29.99
Text explanation:
Customer ID 1 has 2 subscriptions with different prices. The first one starts from 1 January 2020 until December 2020, second one from 3 February 2021 to 5 August 2020.
Customer ID 2 has only 1 subscription, from December 2019 to November 2020.
I want each row equals 1 customer ID, 1 month for easier data manipulation.

Answer 1

generate_series() generates the sequence of dates that you need. However, it is tricky to get the date arithmetic just right for your results.
You seem to want:
select t.cust_id, yyyymm, t.subs_price_p_month
from t cross join lateral
generate_series( date_trunc('month', startd),
date_trunc('month', endd),
interval '1 month'
) gs(yyyymm);
However, if there are multiple rows within the same month, you would get duplicates. This question does not clarify what to do in that case. If you need to handle that case, I would suggest asking a new question.
Here is a db<>fiddle.

Answer 2

Use generate_series with an interval of 1 month and the range from start and end, e.g.:
SELECT
cust_id,
generate_series(start,end,interval '1 month'),
subs_price_p_month
FROM t;
cust_id | generate_series | subs_price_p_month
---------+------------------------+--------------------
1 | 2019-01-01 00:00:00+01 | 50.00
1 | 2019-02-01 00:00:00+01 | 50.00
1 | 2019-03-01 00:00:00+01 | 50.00
1 | 2019-04-01 00:00:00+02 | 50.00
1 | 2019-05-01 00:00:00+02 | 50.00
1 | 2019-06-01 00:00:00+02 | 50.00
1 | 2019-07-01 00:00:00+02 | 50.00
...
Perhaps even formatting the dates to 'Month YYYY' would better display your result set:
SELECT
cust_id,
to_char(generate_series(start_,end_,interval '1 month'),'Month YYYY')
subs_price_p_month
FROM t;
cust_id | to_char | subs_price_p_month
---------+----------------+--------------------
1 | January 2019 | 50.00
1 | February 2019 | 50.00
1 | March 2019 | 50.00
1 | April 2019 | 50.00
1 | May 2019 | 50.00
1 | June 2019 | 50.00
1 | July 2019 | 50.00
...
Demo: db<>fiddle