MySQL 8.0

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

CREATE TABLE test (planning_start_date DATE,
planning_end_date DATE,
quantity INT)
INSERT INTO test VALUES
('2019-03-01', '2019-03-05',1500),
('2019-04-01', '2019-04-03', 300)

Records: 2  Duplicates: 0  Warnings: 0


WITH RECURSIVE cte AS (
    SELECT planning_start_date AS date, 
           planning_end_date, 
           quantity / (DATEDIFF(planning_end_date, planning_start_date) + 1) AS daily_qty
    FROM test
    UNION ALL
    SELECT date + INTERVAL 1 DAY, planning_end_date, daily_qty
    FROM cte
    WHERE date < planning_end_date
)
SELECT `date`, daily_qty
FROM cte
ORDER BY `date`


date
daily_qty


2019-03-01
300.0000

2019-03-02
300.0000

2019-03-03
300.0000

2019-03-04
300.0000

2019-03-05
300.0000

2019-04-01
100.0000

2019-04-02
100.0000

2019-04-03
100.0000

date	daily_qty
2019-03-01	300.0000
2019-03-02	300.0000
2019-03-03	300.0000
2019-03-04	300.0000
2019-03-05	300.0000
2019-04-01	100.0000
2019-04-02	100.0000
2019-04-03	100.0000