• 在每一个date分组中，有数条记录，这些记录中，只有满足用户的date是首次（min）的，才被记录
  • 计数神器：sum + case
select date, sum(
	case
	when (user_id, date) in (select user_id, min(date) from login group by user_id)
	then 1
	else 0
	end
) as new
from login
group by date
order by date