Something like this:
SELECT SUM(aantal) as relatief
FROM _dossier i3
WHERE date = (
SELECT date
from (
select date,
dense_rank() over (partition by i4.instantie order by date desc) as rnk
FROM _dossier i4
WHERE i4.instantie = i3.instantie
) t
where rnk = 2;
);
I'm not entirely sure, that i3 is actually visible inside the nested derived table!
You will need to run an explain
to find out if the LIMI/OFFSET query from Wiktor is faster or slower.