[HR] Challenges***

• Basic Join > sub queries in having

select h.hacker_id, h.name, count(c.challenge_id) cnt
from hackers h
left join challenges c
on h.hacker_id = c.hacker_id
group by h.hacker_id, h.name
having cnt = (select count(cc.challenge_id) from challenges cc group by cc.hacker_id order by count(cc.challenge_id) desc limit 1) 
or cnt in (select t.cnt 
           from (select ccc.hacker_id id, count(ccc.challenge_id) cnt 
                 from challenges ccc
                 group by ccc.hacker_id) t 
           group by t.cnt 
           having count(t.id) = 1)
order by cnt desc, h.hacker_id asc