求一个好欧巴帮忙看下，这么段的句子为啥跑出来是空的？

select
    a.os,
    a.country,
    datediff(b.member_gmt_create,a.hp_stat_date) as PP,
    count(distinct a.imeisi) as geshu
from bdl_en_app_wdm_v3_user_track_d a
join idl_en_ent_signin_fdt0 b
on  a.session_id = b.session_id
and a.newly_imeisi is not null
and a.hp_stat_date>='2014-06-20'
and b.hp_stat_date>='2014-06-20'
group by
    a.os,
    a.country,
    datediff(b.member_gmt_create,a.hp_stat_date)
统计app用户 从装机时间到注册时间的差 的用户数量。
就是用户在装机后一天注册的有XX个，后两天注册的有XXX个·····这样