SQL Basic Join - Challenges

 Problem
Julia asked her students to create some coding challenges. Write a query to print the hacker_id, name, and the total number of challenges created by each student. Sort your results by the total number of challenges in descending order. If more than one student created the same number of challenges, then sort the result by hacker_id. If more than one student created the same number of challenges and the count is less than the maximum number of challenges created, then exclude those students from the result.

Input Format

The following tables contain challenge data:

• Hackers: The hacker_id is the id of the hacker, and name is the name of the hacker.
• Challenges: The challenge_id is the id of the challenge, and hacker_id is the id of the student who created the challenge.

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Sample Input 0
Hackers Table: Challenges Table:
Sample Output 0

21283 Angela 6
88255 Patrick 5
96196 Lisa 1

Sample Input 1
Hackers Table: Challenges Table:
Sample Output 1

12299 Rose 6
34856 Angela 6
79345 Frank 4
80491 Patrick 3
81041 Lisa 1

Explanation

For Sample Case 0, we can get the following details:

Students and both created challenges, but the maximum number of challenges created is so these students are excluded from the result.
For Sample Case 1, we can get the following details:

Students and both created challenges. Because is the maximum number of challenges created, these students are included in the result.

Answer
SELECT h.hacker_id,
       h.NAME,
       Count(challenge_id) AS c_count
FROM   hackers h
       LEFT JOIN challenges c
              ON h.hacker_id = c.hacker_id
GROUP  BY h.hacker_id,
          h.NAME
HAVING c_count = (SELECT Max(t_count)
                  FROM   (SELECT Count(challenge_id) AS t_count
                          FROM   challenges
                          GROUP  BY hacker_id) AS tbl)
        OR c_count IN (SELECT t_count
                       FROM   (SELECT Count(challenge_id) AS t_count
                               FROM   challenges
                               GROUP  BY hacker_id) AS tbl
                       GROUP  BY t_count
                       HAVING Count(t_count) = 1)
ORDER  BY c_count DESC,
          h.hacker_id ASC