select 港コード,港名,入国者数,出国者数,入国者数-出国者数 as 差分
from (select 
port.PORT_CODE as "港コード",
port.PORT_NAME as "港名",
SUM(case imi.KIND_CODE when "110" then AMT else 0 end) as "入国者数",
SUM(case imi.KIND_CODE when "120" then AMT else 0 end) as "出国者数"
from IMMIGRATION as imi
inner join PORT as port
on imi.PORT_CODE=port.PORT_CODE
group by imi.PORT_CODE)
where 差分>0
order by 差分 desc,港コード desc