PostgreSQL | Self-join | Recommenders

-- https://pgexercises.com/questions/joins/self.html
/*
-- SHOW EVERYTHING first FOR FULL 'OBVIOUSITY':
select m.memid, m.firstname, m.surname, m.recommendedby, r.firstname, r.surname
from cd.members m
join cd.members r
on r.memid = m.recommendedby
order by 2 asc

-- THEN REDUCE TO THE ANSWER:
*/
select distinct r.firstname, r.surname
from cd.members m
join cd.members r
on r.memid = m.recommendedby
order by 2 asc