seq_month_value.sql

-- testing with a table "foo" like following
--
-- start_ymd |end_ymd   |val
-- ----------+----------+---
-- 2019-12-01|2020-02-29|bar
-- 2020-03-01|2020-05-31|qux
--
-- to convert it like
--
-- 201912|bar
-- 202001|bar
-- 202002|bar
-- 202003|qux
-- 202004|qux
-- 202005|qux
--
-- SQLite
select year.column1 * 100 + month.column1, foo.val
from (values(2019), (2020)) year,
     (values(1), (2), (3), (4), (5), (6), (7), (8), (9), (10), (11), (12)) month,
     foo
where year.column1 * 100 + month.column1
        between cast(substr(replace(foo.start_ymd, '-', ''), 1, 6) as decimal)
            and cast(substr(replace(foo.end_ymd, '-', ''), 1, 6) as decimal)
order by 1;

-- PostgreSQL (not tested)
select year.n * 100 + month.n, foo.val
from (values(2019), (2020)) year(n),
     (values(1), (2), (3), (4), (5), (6), (7), (8), (9), (10), (11), (12)) month(n),
     foo
where year.n * 100 + month.n
        between cast(substring(replace(foo.start_ymd, '-', ''), 1, 6) as decimal)
            and cast(substring(replace(foo.end_ymd, '-', ''), 1, 6) as decimal)
order by 1;