Complex SQL Query

A.Complex SQL Query.md

Complex SQL Query

See Challenges on HackerRank

Solution by James Everard

query.sql

/* these are the columns we want to output */
select c.hacker_id, h.name ,count(c.hacker_id) as c_count

/* this is the join we want to output them from */
from Hackers as h
    inner join Challenges as c on c.hacker_id = h.hacker_id

/* after they have been grouped by hacker */
group by c.hacker_id, h.name 

/* but we want to be selective about which hackers we output */
/* having is required (instead of where) for filtering on groups */
having 

    /* output anyone with a count that is equal to... */
    c_count = 
        /* the max count that anyone has */
        (SELECT MAX(temp1.cnt)
        from (SELECT COUNT(hacker_id) as cnt
             from Challenges
             group by hacker_id
             order by hacker_id) temp1)

    /* or anyone whose count is in... */
    or c_count in 
        /* the set of counts... */
        (select t.cnt
         from (select count(*) as cnt 
               from challenges
               group by hacker_id) t
         /* whose group of counts... */
         group by t.cnt
         /* has only one element */
         having count(t.cnt) = 1)

/* finally, the order the rows should be output */
order by c_count DESC, c.hacker_id;