SELECT
    PORT_CODE AS "港コード",
    PORT_NAME AS "港名",
    IN_AMT AS "入国者数",
    OUT_AMT AS "出国者数",
    IN_AMT-OUT_AMT AS "差分"
FROM 
    (SELECT PORT.PORT_CODE, PORT.PORT_NAME, GRP.GROUP_CODE, SUM(CASE WHEN IM.KIND_CODE=110 THEN IM.AMT ELSE 0 END) AS IN_AMT, SUM(CASE WHEN IM.KIND_CODE=120 THEN IM.AMT ELSE 0 END) AS OUT_AMT
     FROM IMMIGRATION AS IM
     INNER JOIN PORT ON (IM.PORT_CODE = PORT.PORT_CODE)
     INNER JOIN GRP ON (IM.GROUP_CODE = GRP.GROUP_CODE)
     GROUP BY PORT.PORT_CODE)
WHERE "差分" > 0
ORDER BY "差分" DESC, PORT_CODE DESC