SELECT
IMM.PORT_CODE AS 港コード,
PORT.PORT_NAME AS 港名,
CASE IMM.AMT WHEN IMM.KIND_CODE = 110 THEN SUM(AMT) 
ELSE NULL END AS 入国者数,
CASE IMM.AMT WHEN IMM.KIND_CODE = 120 THEN SUM(AMT) 
ELSE NULL END AS 出国者数,
'入国者数' - '出国者数' AS 差分

FROM IMMIGRATION AS IMM

INNER JOIN PORT AS PORT
ON PORT.PORT_CODE = IMM.PORT_CODE

INNER JOIN GRP AS GRP
ON GRP.GROUP_CODE = IMM.GROUP_CODE

WHERE GRP.GROUP_NAME = '外国人'

GROUP BY IMM.PORT_CODE,PORT.PORT_NAME

ORDER BY 差分 DESC,
港コード DESC;