Hard 2

select submission_date ,
(SELECT COUNT(distinct hacker_id)  
FROM Submissions s2  
WHERE s2.submission_date = s1.submission_date AND    
(SELECT COUNT(distinct s3.submission_date) 
FROM Submissions s3 WHERE s3.hacker_id = s2.hacker_id AND s3.submission_date < s1.submission_date) = dateDIFF(s1.submission_date , '2016-03-01')) ,
(SELECT hacker_id  from submissions s2 WHERE s2.submission_date = s1.submission_date 
group by hacker_id order by count(submission_id) desc , hacker_id limit 1) as shit,
(SELECT name from hackers WHERE hacker_id = shit)
from 
(SELECT distinct submission_date from submissions) s1
group by submission_date