Najděte mezery v sekvenci v SQL bez vytváření dalších tabulek

S moderním SQL , lze to snadno provést pomocí funkcí oken :

select invoice_number + 1 as gap_start, 
       next_nr - 1 as gap_end
from (
  select invoice_number, 
         lead(invoice_number) over (order by invoice_number) as next_nr
  from invoices
) nr
where invoice_number + 1 <> next_nr;

SQLFiddle:http://sqlfiddle.com/#!15/1e807/1