SELECT enter.PORT_CODE as 港コード,
PORT.PORT_NAME as 港名,
enter.AMT as 入国者数,
exit.AMT as 出国者数,
enter.AMT - exit.AMT as 差分
FROM 
(
SELECT * FROM IMMIGRATION 
JOIN GRP on IMMIGRATION.GROUP_CODE = GRP.GROUP_CODE
WHERE KIND_CODE = 110 AND GRP.GROUP_NAME = '外国人'
) as enter

JOIN PORT on PORT.PORT_CODE = enter.PORT_CODE

JOIN 
(
SELECT * FROM IMMIGRATION 
JOIN GRP on IMMIGRATION.GROUP_CODE = GRP.GROUP_CODE
WHERE KIND_CODE = 120 AND GRP.GROUP_NAME = '外国人'
) as exit 
on PORT.PORT_CODE = exit.PORT_CODE
WHERE enter.AMT > exit.AMT
ORDER BY 差分 desc, enter.PORT_CODE desc