【hive】 rollup的空值过滤

rollup 用于hive在group by的顺序分组聚合，举例如下：

select  mac, stat_date,
        avg(cast(cpuOccupancyRate as int)) cpu_avg
from bidata.t_ods_pp_uplink_status_id
where stat_date>=20220119 and stat_date<=20220119
group by rollup(mac,stat_date)

输出结果：

以上内容可以看出，mac和stat_date会分别进行聚合，输出值为NULL，有时候不希望输出mac为NULL的结果，自然想到通过where mac is not null 进行过滤，实际验证

select * from
(
select  mac, stat_date,
        avg(cast(cpuOccupancyRate as int)) cpu_avg
from bidata.t_ods_pp_uplink_status_id
where stat_date>=20220119 and stat_date<=20220119
group by rollup(mac,stat_date)
) as t1 
where mac is not null

结果如下

为什么没有完成过滤呢？难道是NULL 不是空？是真的字符串值为NULL吗？经过验证，where mac!=‘NULL’，结果依然如上，满脸问号？？？？
经过反复验证，解决方案如下：

select * from
(
select  NVL(mac,'mac') as mac, NVL(stat_date,'stat') as stat_date,
        avg(cast(cpuOccupancyRate as int)) cpu_avg
from bidata.t_ods_pp_uplink_status_id
where stat_date>=20220119 and stat_date<=20220119
group by rollup(mac,stat_date)
) as t1 
where mac != 'mac'

结果如下

为什么？？？？还没找到原因，知道的同学求解答！！