SELECT T.港コード, 
    T.港名,
    T.入国者数, 
    T.出国者数,
    T.入国者数 - T.出国者数 AS 差分
FROM 
    (SELECT IMMIGRATION.PORT_CODE AS 港コード, 
    PORT.PORT_NAME AS 港名,
    SUM(CASE WHEN IMMIGRATION.GROUP_CODE = "120" AND IMMIGRATION.KIND_CODE = "110" THEN AMT ELSE 0 END) AS 入国者数, 
    SUM(CASE WHEN IMMIGRATION.GROUP_CODE = "120" AND IMMIGRATION.KIND_CODE = "120" THEN AMT ELSE 0 END) AS 出国者数
    FROM IMMIGRATION
        INNER JOIN PORT ON IMMIGRATION.PORT_CODE = PORT.PORT_CODE
        GROUP BY IMMIGRATION.PORT_CODE
    ) AS T
ORDER BY 差分 DESC, T.港コード DESC;