SELECT
    I1.PORT_CODE AS '港コード'
    ,P.PORT_NAME AS '港名'
    ,I1.AMT AS '入国者数'
    ,I2.AMT AS '出国者数'
    ,(I1.AMT - I2.AMT) AS '差分'
FROM
    IMMIGRATION AS I1
INNER JOIN
    IMMIGRATION AS I2
ON 
    I1.PORT_CODE = I2.PORT_CODE
    AND
    I1.KIND_CODE < I2.KIND_CODE
INNER JOIN
    PORT AS P
ON
    I1.PORT_CODE = P.PORT_CODE
INNER JOIN
    GRP AS G
ON
    I1.GROUP_CODE = G.GROUP_CODE
WHERE
    G.GROUP_NAME = '外国人'
    AND
    差分 > 0
ORDER BY
    差分 DESC, 港コード DESC