PGExercises.com | PostgreSQL | Distinct on

-- https://pgexercises.com/questions/aggregates/nbooking.html

with t1 as(
  select surname, firstname, m.memid, starttime
  from cd.members m
  join cd.bookings b
  on b.memid = m.memid
  where starttime::DATE >= '2012-09-01'
  group by 1,2,3,4 -- removing group by produces wrong data in the next step
  order by 3 asc
)

select distinct on (3) *
from t1