MySQL 8.0

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

CREATE TABLE events (
  start_date DATE,
  end_date DATE,
  event VARCHAR(10)
);
INSERT INTO events VALUES
  ('2019-01-01', '2019-01-04', 'A'),
  ('2019-01-02', '2019-01-03', 'B'),
  ('2019-01-01', '2019-01-06', 'C');
SELECT date_value, COUNT(*)
FROM (
    SELECT ADDDATE('1970-01-01', t4 * 10000 + t3 * 1000 + t2 * 100 + t1 * 10 + t0) AS date_value
    FROM
        (SELECT 0 t0 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) t0,
        (SELECT 0 t1 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,
        (SELECT 0 t2 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,
        (SELECT 0 t3 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) t3,
        (SELECT 0 t4 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) t4
) calendar INNER JOIN events ON calendar.date_value BETWEEN events.start_date AND events.end_date
GROUP BY date_value


date_value
COUNT(*)


2019-01-01
2

2019-01-02
3

2019-01-03
3

2019-01-04
2

2019-01-05
1

2019-01-06
1

date_value	COUNT(*)
2019-01-01	2
2019-01-02	3
2019-01-03	3
2019-01-04	2
2019-01-05	1
2019-01-06	1