Project 2, 3rd question doubt

In the givern task, " Contest Winner Declaration: The team has organized a contest where the user with the most likes on a single photo wins" I have got the user_id with the most likes. But unable to get the username with the code. As i can not add username to group-by clause. Please help me with this ASAP. code - select l.photo_id, count(*) as max from likes l join users u on l.user_id = u.id group by photo_id order by max desc limit 1;