select A.PORT_CODE as 港コード, B.PORT_NAME as 港名,
A.入国者数, A.出国者数,
A.入国者数 - A.出国者数 as 差分
from 
(select 
PORT_CODE ,
sum(case when  KIND_CODE ="110" and GROUP_CODE = "120" then AMT else 0 end) as 入国者数,
sum(case when  KIND_CODE ="120" and GROUP_CODE = "120" then AMT else 0 end) as 出国者数
from IMMIGRATION 
group by PORT_CODE
) A

inner join PORT B
on A.PORT_CODE = B.PORT_CODE

where 差分 > 0
order by 差分 desc, 港コード desc