1.分析
https://blog.nowcoder.net/n/a2f955514f824bb888f9d7726421e809?f=comment
select login.date,ifnull(n1.new_num,0) from login left join (select l1.date,count(distinct l1.user_id) as new_num from login l1 where l1.date = (select min(date) from login where user_id=l1.user_id) group by l1.date) n1 on login.date = n1.date group by login.date order by login.date