代码正在配置单元上运行：

select day,count(mdn)*5 as number from
(select distinct a.mdn,a.day from 
flow a
left outer join
flow b
on a.day=date_add(b.day,-1) and a.mdn=b.mdn
left outer join
flow c
on a.day=date_add(c.day,-2) and a.mdn=c.mdn
left outer join
flow d
on a.day=date_add(d.day,-3) and a.mdn=d.mdn
where b.mdn is null  and c.mdn is null  and d.mdn is null)t 
group by day

代码的逻辑是选择一个今天的mdn，这个mdn在未来三天内没有出现，然后计算mdn的个数，但是这个代码的效率很低，因为使用同一个大表流进行了三次连接。如何高效简化？