WITH tbl1 AS (
SELECT i.PORT_CODE AS '港コード',
    p.PORT_NAME AS '港名',
    i.AMT AS 入国者数
FROM IMMIGRATION AS i
JOIN PORT AS p ON i.PORT_CODE=p.PORT_CODE
JOIN GRP AS g ON i.GROUP_CODE=g.GROUP_CODE
WHERE i.KIND_CODE='110' AND g.GROUP_NAME='外国人'
),
tbl2 AS (
SELECT i.PORT_CODE AS '港コード',
    p.PORT_NAME AS '港名',
    i.AMT AS 出国者数
FROM IMMIGRATION AS i
JOIN PORT AS p ON i.PORT_CODE=p.PORT_CODE
JOIN GRP AS g ON i.GROUP_CODE=g.GROUP_CODE
WHERE i.KIND_CODE='120' AND g.GROUP_NAME='外国人'
)
SELECT tbl1.港コード,
    tbl1.港名,
    tbl1.入国者数,
    tbl2.出国者数,
    tbl1.入国者数-tbl2.出国者数 AS '差分'
FROM tbl1
JOIN tbl2 ON tbl1.港コード=tbl2.港コード
WHERE tbl1.入国者数>tbl2.出国者数
ORDER BY 差分 DESC, tbl1.港コード DESC;