题解 | 思路：当天登陆日期是否为第一次的登陆日期

# 笨办法
select t1.date, sum(if(t1.date = t2.date,1,0)) as new
from login t1
left join
(select user_id, min(`date`) `date`
from login
group by user_id) t2
on t1.user_id = t2.user_id
group by t1.date