datefrom DATE,

datetill DATE,

amount DECIMAL(10, 2));

INSERT INTO test (datefrom, datetill, amount) VALUES

('2020-01-10','2020-02-15',1000),

('2019-01-10','2020-02-15',1000);

SELECT * FROM test;

datefrom,

datetill,

amount,

monthstart,

monthfinish,

amount * (DATEDIFF(LEAST(datetill, monthfinish), GREATEST(datefrom, monthstart)) + 1) / (DATEDIFF(datetill, datefrom) + 1) monthamount

FROM ( SELECT test.*,

(test.datefrom - INTERVAL DAY(test.datefrom) - 1 DAY) + INTERVAL numbers.num MONTH monthstart,

LAST_DAY((test.datefrom - INTERVAL DAY(test.datefrom) - 1 DAY) + INTERVAL numbers.num MONTH) monthfinish

FROM test

JOIN ( SELECT t1.num*10+t2.num num

FROM (SELECT 0 num UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t1

JOIN (SELECT 0 num UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t2

) numbers

HAVING monthstart <= test.datetill

AND monthfinish >= test.datefrom

) subquery

ORDER BY id, monthstart;