This one was difficult. I found a discussion that answered a lot of problems I had with this one. This is what they are really looking for. pretty easy except for the "tracking the people that did it everyday" part. I had to look for help and then ended up creating a buffer to count the everyday people. I had to look at it several times to figure out a workable solution. You have to iterate every day sequentially, and update the everyday people buffer. Once you can nest that in there, it's not so hard.

"select Submission_date, Number of hackers who have submitted every day so far, Hacker_id of the hacker with the most submits on this day, (regardless of which other days they submitted) Their_name