WITH foreigner_table AS (
  SELECT
    PORT_CODE,
    sum(
      CASE
        WHEN kind_code = '110' THEN amt
        ELSE 0
      END
    ) AS 入国者数,
    sum(
      CASE
        WHEN kind_code = '120' THEN amt
        ELSE 0
      END
    ) AS 出国者数
  FROM
    IMMIGRATION
    LEFT JOIN GRP USING(group_code)
  WHERE
    group_name = '外国人'
  group by port_code
)
SELECT
  port_code AS 港コード,
  port_name AS 港名,
  入国者数,
  出国者数,
  入国者数 - 出国者数 AS 差分
FROM
  foreigner_table
  LEFT JOIN PORT USING(PORT_CODE)
WHERE
 入国者数 - 出国者数 >0
ORDER BY
  差分 DESC,
  港コード DESC