SELECT PT.PORT_CODE AS 港コード
,PT.PORT_NAME AS 港名
,IMN.AMT AS 入国者数
,IMNOUT.AMT AS 出国者数
,(IMN.AMT - IMNOUT.AMT) AS 差分
FROM IMMIGRATION AS IMN
LEFT JOIN PORT AS PT
       ON IMN.PORT_CODE = PT.PORT_CODE
INNER JOIN IMMIGRATION AS IMNOUT
        ON IMNOUT.GROUP_CODE =IMN.GROUP_CODE
        AND IMNOUT.PORT_CODE =IMN.PORT_CODE
        AND IMNOUT.KIND_CODE='120'
        AND IMNOUT.GROUP_CODE='120'

WHERE IMN.KIND_CODE='110'
AND IMNOUT.GROUP_CODE='120'
AND 差分>0

GROUP BY PT.PORT_CODE
ORDER BY 差分 DESC,港コード DESC