比较通俗易懂的解法#case when，不用表连接

select dt,count(dt) dau,round(sum(judge)/count(dt),2) uv_new_ratio
from
    (select uid,in_time dt,
    case when (uid,in_time) in 
        (select uid,date(min(in_time)) from tb_user_log group by uid) #所有用户最早活跃时间
         then 1 else 0 end judge 
         #表示：如果等于最早活跃时间，则赋值1，否则赋值0，用于体现新用户。
    from 
         (select uid,date(in_time) in_time from tb_user_log
         union #如果进入时间离开时间跨天了，在两天里都记为该用户活跃过.利用union去重
         select uid,date(out_time) in_time from tb_user_log
          ) AS new
    ) AS n
group by dt
order by dt