create table `tableid` (`id` int);

insert into `tableid` (`id`) values
(1),(2),(3),(4),(5),   (7),(8),(9),(10),(11),(12),  (14);

select
   R.`number`
from

  (
  select 1000*t4.n + 100*t3.n + 10*t1.n + t2.n + 1 as `number`
  from
  (
    (select 0 n union select 1 union select 2 union select 3 union select 4
    union select 5 union select 6 union select 7 union select 8 union select 9 ) t1
    cross join
    (select 0 n union select 1 union select 2 union select 3 union select 4
    union select 5 union select 6 union select 7 union select 8 union select 9 ) t2
    cross join
    (select 0 n union select 1 union select 2 union select 3 union select 4
    union select 5 union select 6 union select 7 union select 8 union select 9) t3
    cross join
    (select 0 n union select 1 union select 2 union select 3 union select 4
    union select 5 union select 6 union select 7 union select 8 union select 9) t4
    )
  ) R

left outer join `tableid` t on t.`id` = R.`number`
where
  R.`number` <2501
  and t.`id` is null
order by `number`

SELECT seq FROM seq_1_to_2501
left join yourtable on seq=youridcol
where seq is null

SELECT seq FROM seq_1_to_2501
left join yourtable on seq=youridcol
where seq is null