SELECT MONTH(dt) as m, DAY(dt) as d, COUNT(*) as cnt
FROM (
SELECT '2011-10-11 10:00:00' as dt
UNION ALL
SELECT '2011-10-11 20:00:00' as dt
UNION ALL
SELECT '2011-10-12 10:00:00' as dt
UNION ALL
SELECT '2011-10-12 20:00:00' as dt
UNION ALL
SELECT '2011-10-13 10:00:00' as dt
UNION ALL
SELECT '2011-10-13 20:00:00' as dt
UNION ALL
SELECT '2011-11-13 10:00:00' as dt
UNION ALL
SELECT '2011-11-13 20:00:00' as dt
) as dates
GROUP BY m, d;
Результат:
+------+------+-----+
| m | d | cnt |
+------+------+-----+
| 10 | 11 | 2 |
| 10 | 12 | 2 |
| 10 | 13 | 2 |
| 11 | 13 | 2 |
+------+------+-----+