select
    max(district_code) as district_code,
    case when coalesce(district_name,'')='' then '不明' else district_name end as district_name,
    sum(total_amt) as total_amt,
    sum(male_amt) as female_amt,
    sum(female_amt) as female_amt
from
    population
group by
    case when coalesce(district_name,'')='' then '不明' else district_name end