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",
  sum(CASE WHEN KIND_CODE = "110" THEN AMT ELSE -AMT END) AS "差分"
FROM IMMIGRATION
INNER JOIN PORT ON IMMIGRATION.PORT_CODE = PORT.PORT_CODE
INNER JOIN GRP ON IMMIGRATION.GROUP_CODE = GRP.GROUP_CODE
WHERE GRP.GROUP_NAME = "外国人"
GROUP BY PORT.PORT_CODE
HAVING sum(CASE WHEN KIND_CODE = "110" THEN AMT ELSE -AMT END) > 0
ORDER BY sum(CASE WHEN KIND_CODE = "110" THEN AMT ELSE -AMT END) DESC;