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 when IMMIGRATION.KIND_CODE = 110
   then IMMIGRATION.AMT end) as 入国者数
, SUM(CASE when IMMIGRATION.KIND_CODE = 120
   then IMMIGRATION.AMT end) as 出国者数
from IMMIGRATION
inner join PORT
on IMMIGRATION.PORT_CODE = PORT.PORT_CODE
inner join GRP
on IMMIGRATION.GROUP_CODE = GRP.GROUP_CODE
where GRP.GROUP_NAME = "外国人" 
group by PORT.PORT_CODE, PORT.PORT_NAME
)
order by 差分 desc