exists ，not exists改写成in 和 not in 时，要注意null值的情况，会影响结果

---not  exists

select count(distinct a.pos_mer_id)
from tests a
where a.stat_flag='C' and not exists
(select 1 from
hfods.tests b
where b.stat_flag='N' and b.pos_mer_id=a.pos_mer_id);

改写成not in

select count(distinct a.pos_mer_id)

from tests a
where a.stat_flag='C' and a.pos_mer_id not in
(select b.pos_mer_id from
tests b
where b.stat_flag='N' and b.pos_mer_id is not null)