select * from (
select 
PORT_CODE "港コード",
PORT_NAME "港名",
sum(case when KIND_CODE = '110' then AMT else 0 end) "入国者数",
sum(case when KIND_CODE = '120' then AMT else 0 end) "出国者数",
sum(case when KIND_CODE = '110' then AMT else 0 end) - sum(case when KIND_CODE = '120' then AMT else 0 end) "差分"
from IMMIGRATION
left join GRP using (GROUP_CODE)
left join PORT using (PORT_CODE)
where GROUP_NAME = '外国人'
group by 1,2
) where "差分"> 0
order by 5 desc, 1 desc