MySQL obtiene ID faltantes de la tabla

SELECT a.id+1 AS start, MIN(b.id) - 1 AS end
    FROM testtable AS a, testtable AS b
    WHERE a.id < b.id
    GROUP BY a.id
    HAVING start < MIN(b.id)

Espero que este enlace también ayude http://www.codediesel.com/mysql /sequence-gaps-in-mysql/