SELECT
  *,
  入国者数 - 出国者数 AS 差分
FROM (SELECT
  PORT.PORT_CODE AS 港コード,
  PORT.PORT_NAME AS 港名,
  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 PORT
LEFT JOIN IMMIGRATION ON PORT.PORT_CODE = IMMIGRATION.PORT_CODE
WHERE IMMIGRATION.GROUP_CODE = "120"
GROUP BY PORT.PORT_CODE
)
WHERE 入国者数 > 出国者数
ORDER BY 差分 DESC, 港コード DESC