SELECT 
a.港コード,
a.港名,
a.入国者数,
a.出国者数,
a.入国者数 - a.出国者数 AS 差分
FROM
(
    SELECT
    IMMIGRATION.PORT_CODE AS 港コード,
    PORT.PORT_NAME AS 港名,
    (SELECT
     AMT
     FROM
     IMMIGRATION b
     WHERE
     GROUP_CODE = '120' AND KIND_CODE = '110' AND IMMIGRATION.PORT_CODE = b.PORT_CODE
    ) AS 入国者数,
    (SELECT
     AMT
     FROM
     IMMIGRATION c
     WHERE
     GROUP_CODE = '120' AND KIND_CODE = '120' AND IMMIGRATION.PORT_CODE = c.PORT_CODE
    ) AS 出国者数
    FROM PORT
    LEFT JOIN IMMIGRATION ON PORT.PORT_CODE = IMMIGRATION.PORT_CODE
    GROUP BY 港コード
)a
WHERE 差分 > 0
GROUP BY 港コード
ORDER BY 差分 DESC,港コード DESC