Старайтесь держать плотный ранг:
with tt as (
select 4 id, date '2018-03-10' due, 335.75 amt from dual union all
select 3 id, date '2018-04-10' due, 334.75 amt from dual union all
select 1 id, date '2018-05-10' due, 333.75 amt from dual union all
select 2 id, date '2018-06-10' due, 332.75 amt from dual
)
select min(due) least_due,
min(amt) keep (dense_rank first order by due) amt_for_least_due,
sum(amt) sum_amt
from tt