MySQL: To exclude some results based on specified conditions - mysql

Question

I am stuck at some problem. The problem has two tables, namely hackers and challenges.
Here is the schema:
Hackers(hacker_id: int, name string)
Challenges(challenge_id: int, hacker_id int)
I am trying to 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.
Here is my query:
select hackers.hacker_id ,
hackers.name ,
count(challenges.challenge_id) as challenges_created
from
hackers left join challenges
on
hackers.hacker_id = challenges.hacker_id
having
count(challenges_created) >= max(challenges_created)
order by challenges_created desc, hackers.hacker_id asc
I am getting wrong output!
Please tell me where I am getting wrong!
Sample Input :
Hackers Table:image
Challengers Table :image
Sample Output:
21283 Angela 6
88255 Patrick 5
96196 Lisa 1

Answer 1

It's a crazy requirement to solve in mysql only. With CTE (not supported by mysql) you could reuse your query:
with tmp as (
select h.hacker_id, h.name, count(1) as challenges_created
from hackers h
left join challenges c on c.hacker_id = h.hacker_id
group by h.hacker_id, h.name
order by challenges_created desc, h.hacker_id asc
), max_challenges_created as (
select max(challenges_created) as challenges_created
from tmp
), count_per_challenges_created as (
select challenges_created, count(1) as c
from tmp
group by challenges_created
)
select *
from tmp
cross join max_challenges_created m
left join count_per_challenges_created c on c.challenges_created = tmp.challenges_created
where tmp.challenges_created = m.challenges_created
or c.c = 1
http://rextester.com/XGYQ11641
In MySQL you can't even use a temporary table, because you couldn't use it multiple times in one query. So you have to copy&paste the same query three times (and hope that mysql will execute it only once using cache):
select *
from (
select h.*, count(1) as challenges_created
from hackers h
left join challenges c on c.hacker_id = h.hacker_id
group by h.hacker_id
order by challenges_created desc, h.hacker_id
) tmp
cross join (
select max(challenges_created) as challenges_created
from (
select h.*, count(1) as challenges_created
from hackers h
left join challenges c on c.hacker_id = h.hacker_id
group by h.hacker_id
order by challenges_created desc, h.hacker_id asc
) tmp
) m
left join (
select challenges_created, count(1) as c
from (
select h.*, count(1) as challenges_created
from hackers h
left join challenges c on c.hacker_id = h.hacker_id
group by h.hacker_id
order by challenges_created desc, h.hacker_id asc
) tmp
group by challenges_created
) c on c.challenges_created = tmp.challenges_created
where tmp.challenges_created = m.challenges_created
or c.c = 1
http://rextester.com/IDUMRB49795

Answer 2

Try start solving smallest part of the query and keep joining them.
You can try my code which is in SQL(Oracle)
SELECT h.hacker_id,
h.name,
COUNT(c.challenge_id) AS challenge_count
FROM Hackers h
JOIN Challenges c ON c.hacker_id = h.hacker_id
GROUP BY h.hacker_id, h.name
HAVING challenge_count =
(SELECT COUNT(challenge_id) AS count_max
FROM Challenges
GROUP BY hacker_id
ORDER BY count_max DESC limit 1)
OR challenge_count IN
(SELECT DISTINCT c_compare AS c_unique
FROM (SELECT h2.hacker_id,
h2.name,
COUNT(challenge_id) AS c_compare
FROM Hackers h2
JOIN Challenges c ON c.hacker_id = h2.hacker_id
GROUP BY h2.hacker_id, h2.name) counts
GROUP BY c_compare
HAVING COUNT(c_compare) = 1)
ORDER BY challenge_count DESC, h.hacker_id;

Answer 3

try this it's very simple....
select
h.hacker_id,
h.name
from hackers h, difficulty d, challenges c, submissions s
where
h.hacker_id=s.hacker_id
and s.challenge_id=c.challenge_id
and d.difficulty_level=c.difficulty_level
and d.score=s.score
group by h.hacker_id, h.name
having count(*)>1
ORDER BY COUNT(*) DESC, h.hacker_id;

Answer 4

This is not the real answer, but the start point for you:
http://sqlfiddle.com/#!9/6910c/5
SELECT h.hacker_id ,
h.name ,
COUNT(challenges.challenge_id) as challenges_created
FROM hackers h
LEFT JOIN challenges
ON h.hacker_id = challenges.hacker_id
GROUP BY h.hacker_id
#HAVING challenges_created >= max(challenges_created)
ORDER BY challenges_created desc, h.hacker_id asc
I've commented out your HAVING clause, because can not understand what you are trying to achieve.
Explain please why do you expect only 3 rows in result? Why Rose and Frank should be excluded from the result set?

Answer 5

The solution is easy when you use multiple joins on the necessary tables. I like to group results as derived tabled for clarity but it can be done without those derived tables too.
select hckid, nme
from
(select h.name as nme, h.hacker_id as hckid, cnt
from (select s.hacker_id as hackerid, count(d.score) as cnt
from submissions s
join challenges c
on c.challenge_id = s.challenge_id
join difficulty d
on c.difficulty_level = d.difficulty_level
where d.score = s.score
group by s.hacker_id
having count(d.score) > 1) ft
join hackers h
on ft.hackerid = h.hacker_id) t
order by t.cnt desc, t.hckid asc

Answer 6

select hacker_id, name, tmp.challenges_created
from (
select h.*, count(1) as challenges_created
from hackers h
left join challenges c on c.hacker_id = h.hacker_id
group by h.hacker_id, h.name
order by challenges_created desc, h.hacker_id
) tmp
cross join (
select max(challenges_created) as challenges_created
from (
select h.*, count(1) as challenges_created
from hackers h
left join challenges c on c.hacker_id = h.hacker_id
group by h.hacker_id, h.name
order by challenges_created desc, h.hacker_id asc
) tmp
) m
left join (
select challenges_created, count(1) as c
from (
select h.*, count(1) as challenges_created
from hackers h
left join challenges c on c.hacker_id = h.hacker_id
group by h.hacker_id, h.name
order by challenges_created desc, h.hacker_id asc
) tmp
group by challenges_created
) c on c.challenges_created = tmp.challenges_created
where tmp.challenges_created = m.challenges_created
or c.c = 1

Answer 7

SELECT a.hacker_id, a.name, b.id
FROM Hackers a
INNER JOIN (SELECT hacker_id, id
FROM (SELECT hacker_id, count(challenge_id) AS id
FROM Challenges
GROUP BY hacker_id) x
WHERE id = 50
UNION ALL
SELECT hacker_id,id
FROM (SELECT hacker_id, count(challenge_id) AS id
FROM Challenges
GROUP BY hacker_id) y
WHERE id IN (SELECT id
FROM (SELECT hacker_id, count(challenge_id) AS id
FROM Challenges
GROUP BY hacker_id) x
GROUP BY id
HAVING count(*) = 1)
) b ON a.hacker_id = b.hacker_id
ORDER BY id DESC, hacker_id

Answer 8

This is a question in hackerranck.com you can check it here [link]
in this site you can pass answer by many sql DBMS (oracale,maysql,sqlserver, and DB2)
It's Work very well for SQL-SERVER
with t
as(
Select s.hacker_id ,
s.score,d.Difficulty_level as Difficulty_level,
c.Challenge_id
From submissions s
inner join Challenges c on c.Challenge_id = s.Challenge_id
inner join Difficulty d on d.Difficulty_level = c.Difficulty_level and d.score = s.score
)
Select t.hacker_id, h.name
From t inner join hackers h ON t.hacker_id=h.hacker_id
Where t.score in (Select score from Difficulty)
Group by t.hacker_id,h.name
Having count(t.Challenge_id) > 1
Order by count(t.Challenge_id) desc,t.hacker_id asc

Answer 9

I have done in SQL Server
I have joined 2 different selects , the first one uses rank to determine the ones with rank 1 and the second finds duplicates , so in the join result I filtered the result to have either 1st rank or no duplicates (cnt = 1)
select s.hacker_id, s.name, s.challenge_created from (
select * ,rank() over (order by challenge_created desc) rn from (
select h.hacker_id, name, count(challenge_id) challenge_created from hackers h
inner join challenges c on h.hacker_id = c.hacker_id
group by h.hacker_id, name) as T) as S
inner join (select A.challenge_created , count(1) cnt from (select h.hacker_id, name, count(challenge_id) challenge_created from hackers h
inner join challenges c on h.hacker_id = c.hacker_id
group by h.hacker_id, name) as A group by A.challenge_created) as O
on s.challenge_created = O.challenge_created
where rn = 1 or O.cnt = 1
order by s.challenge_created desc , hacker_id

Answer 10

The below query works fine in mysql
For having clause condition
I have included the record in which the count is either equal to
maximum number of challenges created OR
the number of challenges created is unique
select c.hacker_id,h.name,count(c.challenge_id) as cnt from
challenges c join hackers h
on c.hacker_id = h.hacker_id
group by c.hacker_id,h.name
having cnt =
(select max(counts) from (select count(challenge_id) as counts from challenges group by hacker_id) as countstable1) or
cnt in (select counts from (select count(challenge_id) as counts from challenges group by hacker_id) as countstable2 group by counts having count(*)=1)
order by cnt desc,c.hacker_id