Generating multiple rows from a single row based on dates - sql

Question

I have a database table with a start date and a number of months. How can I transform that into multiple rows based on the number of months?
I want to transform this
Into this:

Answer 1

We can try using a calendar table here, which includes all possible start of month dates which might appear in the expected output:
with calendar as (
select '2017-09-01'::date as dt union all
select '2017-10-01'::date union all
select '2017-11-01'::date union all
select '2017-12-01'::date union all
select '2018-01-01'::date union all
select '2018-02-01'::date union all
select '2018-03-01'::date union all
select '2018-04-01'::date union all
select '2018-05-01'::date union all
select '2018-06-01'::date union all
select '2018-07-01'::date union all
select '2018-08-01'::date
)
select
t.id as subscription_id,
c.dt,
t.amount_monthly
from calendar c
inner join your_table t
on c.dt >= t.start_date and
c.dt < t.start_date + (t.month_count::text || ' month')::interval
order by
t.id,
c.dt;
Demo

Answer 2

This can easily be done using generate_series() in Postgres
select t.id,
g.dt::date,
t.amount_monthly
from the_table t
cross join generate_series(t.start_date,
t.start_date + interval '1' month * (t.month_count - 1),
interval '1' month) as g(dt);

Answer 3

OK, it's very easy to implement this in PostgreSQL, just use generate_series, as below:
select * from month_table ;
id | start_date | month_count | amount | amount_monthly
------+------------+-------------+--------+----------------
1382 | 2017-09-01 | 3 | 38 | 1267
1383 | 2018-02-01 | 6 | 50 | 833
(2 rows)
select
id,
generate_series(start_date,start_date + (month_count || ' month') :: interval - '1 month'::interval, '1 month'::interval)::date as date,
amount_monthly
from
month_table ;
id | date | amount_monthly
------+------------+----------------
1382 | 2017-09-01 | 1267
1382 | 2017-10-01 | 1267
1382 | 2017-11-01 | 1267
1383 | 2018-02-01 | 833
1383 | 2018-03-01 | 833
1383 | 2018-04-01 | 833
1383 | 2018-05-01 | 833
1383 | 2018-06-01 | 833
1383 | 2018-07-01 | 833
(9 rows)

Answer 4

You may not need so many subqueries but this should help you understand how it can be broken down
WITH date_minmax AS(
SELECT
min(start_date) as date_first,
(max(start_date) + (month_count::text || ' months')::interval)::date AS date_last
FROM "your_table"
GROUP BY month_count
), series AS (
SELECT generate_series(
date_first,
date_last,
'1 month'::interval
)::date as list_date
FROM date_minmax
)
SELECT
id as subscription_id,
list_date as date,
amount_monthly as amount
FROM series
JOIN "your_table"
ON list_date <# daterange(
start_date,
(start_date + (month_count::text || ' months')::interval)::date
)
ORDER BY list_date
This should achieve the desired result http://www.sqlfiddle.com/#!17/7d943/1