Isso pode ajudá-lo (um pouco).
select a.id, a.length,
coalesce(a.length -
(select b.length from foo b where b.id = a.id + 1), a.length) as diff
from foo a
select a.id, a.length,
coalesce(a.length -
(select b.length from foo b where b.id = a.id + 1), a.length) as diff
from foo a
