[firebird-support] Firebird get the list with all available id

[radubarbu84]

In a table I have records with id's 2,4,5,8. How can I receive a list with 
values 1,3,6,7. I have tried in this way

SELECT t1.id + 1
FROM table t1
WHERE NOT EXISTS (
    SELECT * 
    FROM table t2
    WHERE t2.id = t1.id + 1
)

but it's not working correctly. It doesn't bring all available positions.

Is it possible without creating another table?