You have a table named tbl with an integer primary key named id, and you need to know what key values between 0 and 999999 are missing in the table.
The simplest solution is an exclusion join from a virtual table of sequential numbers to the target table:

create or replace view v as
select 0 i 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;
select x.i
from (
select a.i*100000 + b.i*10000 + c.i*1000 + d.i*100 + e.i*10 + f.i as i
from v a
join v b
join v c
join v d
join v e
join v f
) x
left join tbl on x.i=tbl.id
where tbl.id is null
order by x.i;