You can use a concept that is frequently referred to as ‘calendar tables’. Here is a good guide on how to create calendar tables in MySql:
-- create some infrastructure
CREATE TABLE ints (i INTEGER);
INSERT INTO ints VALUES (0), (1), (2), (3), (4), (5), (6), (7), (8), (9);
-- only works for 100 days, add more ints joins for more
SELECT cal.date, tbl.data
FROM (
SELECT '2009-06-25' + INTERVAL a.i * 10 + b.i DAY as date
FROM ints a JOIN ints b
ORDER BY a.i * 10 + b.i
) cal LEFT JOIN tbl ON cal.date = tbl.date
WHERE cal.date BETWEEN '2009-06-25' AND '2009-07-01';
You might want to create table cal
instead of the subselect.