select * FROM(
select
PORT.PORT_CODE AS '港コード' ,
PORT_NAME as '港名',
coalesce(nyuukoku.AMT,0) as '入国者数',
coalesce(shukkoku.AMT,0) as '出国者数',
coalesce(nyuukoku.AMT,0) - coalesce(shukkoku.AMT,0)  as '差分'
from PORT
left join 
(select shukkoku.*
 from 
 IMMIGRATION shukkoku
 join GRP on shukkoku.GROUP_CODE = GRP.GROUP_CODE AND shukkoku.KIND_CODE = '120'-- 出国者
 ) shukkoku on PORT.PORT_CODE = shukkoku.PORT_CODE
left join (select  nyuukoku.*
 from 
 IMMIGRATION nyuukoku
 join GRP on nyuukoku.GROUP_CODE = GRP.GROUP_CODE AND nyuukoku.KIND_CODE = '110' --外国人入国者
 where GRP.GROUP_CODE = '120') nyuukoku on PORT.PORT_CODE = nyuukoku.PORT_CODE
 ) kekka
 where kekka.'差分' >= 0
order by kekka.'差分' desc,kekka.'港コード' desc