select 
IM.PORT_CODE as 港コード,
PORT_NAME as 港名,
SUM(CASE
    WHEN KIND_CODE = '110' AND IM.GROUP_CODE = '120' THEN AMT
    ELSE 0
END) as 入国者数,
SUM(CASE
    WHEN KIND_CODE = '120' AND IM.GROUP_CODE = '120' THEN AMT
    ELSE 0
END) as 出国者数,
SUM(CASE
    WHEN KIND_CODE = '110' AND IM.GROUP_CODE = '120' THEN AMT
    ELSE 0
END)-SUM(CASE
    WHEN KIND_CODE = '120' AND IM.GROUP_CODE = '120' THEN AMT
    ELSE 0
END) as 差分


from
    IMMIGRATION as IM
    inner join PORT as PO
    on IM.PORT_CODE = PO.PORT_CODE
    inner join GRP as GR
    on IM.GROUP_CODE = GR.GROUP_CODE
GROUP BY 港コード
HAVING 差分>0

ORDER BY 差分 DESC,
港コード DESC