Mine

SELECT a.bar+1 AS 'Missing From', MIN(b.bar) - 1 AS 'To' FROM foo AS a, foo AS b WHERE a.bar < b.bar GROUP BY a.bar HAVING 'Missing From' < MIN(b.bar) - 1 ORDER BY 1;
+--------------+------+
| Missing From | To   |
+--------------+------+
|            2 |    1 |
|            3 |    3 |
+--------------+------+