Вы можете создать дополнительную таблицу с количеством дней и объединить две таблицы.Вот пример, он работает за указанный месяц -
CREATE TABLE days (
number INT(11) NOT NULL
);
CREATE TABLE sms_subscribers (
id INT(11) NOT NULL AUTO_INCREMENT,
created DATETIME DEFAULT NULL,
PRIMARY KEY (id)
);
INSERT INTO days VALUES
(1), (2), (3), (4), (5), (6), (7), (8), (9), (10),
(11), (12), (13), (14), (15), (16), (17), (18), (19), (20),
(21), (22), (23), (24), (25), (26), (27), (28), (29), (30), (31);
INSERT INTO sms_subscribers VALUES
(1, '2011-08-25 08:35:27'),
(2, '2011-08-11 08:35:30'),
(3, '2011-08-06 08:35:35'),
(4, '2011-06-06 08:35:40'),
(5, '2011-06-19 08:35:47'),
(6, '2011-06-01 08:35:53'),
(7, '2011-08-28 08:36:02'),
(8, '2011-08-25 08:57:58');
SELECT d.number, COUNT(t.created) FROM days d
LEFT JOIN (SELECT * FROM sms_subscribers
WHERE YEAR(created) = 2011 AND MONTH(created) = 8) t -- Specify year and monts here
ON DAYOFMONTH(t.created) = d.number
WHERE
d.number <= DAYOFMONTH(LAST_DAY('2011-08-01')) -- Specify any day in the specified month, it will help to found out max number of days in the month
GROUP BY d.number;
+--------+------------------+
| number | COUNT(t.created) |
+--------+------------------+
| 1 | 0 |
| 2 | 0 |
| 3 | 0 |
| 4 | 0 |
| 5 | 0 |
| 6 | 1 |
| 7 | 0 |
| 8 | 0 |
| 9 | 0 |
| 10 | 0 |
| 11 | 1 |
| 12 | 0 |
| 13 | 0 |
| 14 | 0 |
| 15 | 0 |
| 16 | 0 |
| 17 | 0 |
| 18 | 0 |
| 19 | 0 |
| 20 | 0 |
| 21 | 0 |
| 22 | 0 |
| 23 | 0 |
| 24 | 0 |
| 25 | 2 |
| 26 | 0 |
| 27 | 0 |
| 28 | 1 |
| 29 | 0 |
| 30 | 0 |
| 31 | 0 |
+--------+------------------+