SQL查询案例和group by语句澄清

Question

我对以下查询感到困惑..

select 
    kgfs,
    branch,
    (case when (Overdue_Days_as_on_30Sep2016) between 1 and 30 
        then sum(Total_Cash_Collected_for_the_period)/sum(principaloverdue_as_on_30SEP2016+InterestOverdue_as_on_30SEP2016) 
        else 0 end
    ) as 1to30perc
from 
bietl_archive.quick_cbs_loan_dump_01Oct2016to13Oct2016
group by kgfs,branch 

当我使用这个查询结果时会自动转到其他部分..我做了什么错误..请问你能解释一下我吗