SELECT
    PORT_CODE AS '港コード',
    PORT_NAME AS '港名',
    enter.amt_sum AS '入国者数',
    exit.amt_sum AS '出国者数',
    enter.amt_sum - exit.amt_sum AS '差分'
FROM IMMIGRATION
INNER JOIN PORT USING (PORT_CODE)
INNER JOIN (
    SELECT
        PORT_CODE,
        SUM(AMT) AS amt_sum
    FROM IMMIGRATION
    INNER JOIN GRP USING (GROUP_CODE)
    WHERE
        KIND_CODE = 110
        AND GROUP_CODE = 120
    GROUP BY PORT_CODE
) AS enter USING(PORT_CODE)
INNER JOIN (
    SELECT
        PORT_CODE,
        SUM(AMT) AS amt_sum
    FROM IMMIGRATION
    INNER JOIN GRP USING (GROUP_CODE)
    WHERE 
        KIND_CODE = 120
        AND GROUP_CODE = 120
    GROUP BY PORT_CODE
) AS exit USING(PORT_CODE)
WHERE enter.amt_sum > exit.amt_sum
GROUP BY 港コード
ORDER BY 差分 DESC, PORT_CODE DESC;