题解 | #case when 的使用#

select
    date,
    count(distinct case when (user_id, date) in (
        select user_id, min(date) from login group by user_id
    ) then user_id else null end) new
from login
group by date
order by date

case when 可以在count中做条件语句，distinct 防止在同一天登录了多次，导致多条记录重复计数。