select 
   a.PORT_CODE AS "港コード"
  ,a.PORT_NAME AS "港名"
  ,b1.AMT      AS "入国者数"
  ,b2.AMT      AS "出国者数"
  ,b1.AMT - b2.AMT AS "差分"
from
  PORT a
 ,(select AMT,PORT_CODE from IMMIGRATION x1
                            ,GRP         x2
   where x1.GROUP_CODE = x2.GROUP_CODE
   and   x2.GROUP_CODE = '120'
   and   x1.KIND_CODE  = '110'
  ) b1
 ,(select AMT,PORT_CODE from IMMIGRATION x1
                            ,GRP         x2
   where x1.GROUP_CODE = x2.GROUP_CODE
   and   x2.GROUP_CODE = '120'
   and   x1.KIND_CODE  = '120'
  ) b2
where
    a.PORT_CODE = b1.PORT_CODE
and a.PORT_CODE = b2.PORT_CODE
order by "差分" DESC,a.PORT_CODE DESC