Това е проблем с пропуски и острови. Има различни подходи, но простият използва разлика в номера на реда:
with paaf as (<your first query here>
)
select paaf.assignment_id,
paaf.position_id,
min(paaf.effective_start_date) as effective_start_date,
max(paaf.effective_end_date) as effective_end_date
from (select paaf.*,
row_number() over (order by effective_start_date) as seqnum,
row_number() over (partition by position_id order by effective_start_date) as seqnum_p
from paaf
) paaf
group by (seqnum - seqnum_p), position_id, assignment_id;