SELECT l.id + 1 AS START FROM SEQUENCE AS l LEFT OUTER JOIN SEQUENCE AS r ON l.id + 1 = r.id WHERE r.id IS NULL
SELECT MIN(l.id + 1) AS START FROM SEQUENCE AS l LEFT OUTER JOIN SEQUENCE AS r ON l.id + 1 = r.id WHERE r.id IS NULL
SELECT START, stop FROM ( SELECT m.id + 1 AS START, (SELECT MIN(id) - 1 FROM SEQUENCE AS x WHERE x.id > m.id) AS stop FROM SEQUENCE AS m LEFT OUTER JOIN SEQUENCE AS r ON m.id = r.id - 1 WHERE r.id IS NULL ) AS x WHERE stop IS NOT NULL;
SELECT id, COUNT(*) FROM SEQUENCE GROUP BY id HAVING COUNT(*) > 1;