WITH NYUKOKU AS (

SELECT
I.PORT_CODE  AS "港コード",
P.PORT_NAME   AS  "港名",
SUM(I.AMT)   AS  "入国者数"
--足りなければ足す
--↓JOIN
FROM IMMIGRATION AS I
INNER JOIN PORT AS P
  ON I.PORT_CODE = P.PORT_CODE
  
INNER JOIN GRP AS G
  ON I.GROUP_CODE = G.GROUP_CODE
  
WHERE
I.GROUP_CODE = '110' --入国者だけ

GROUP BY I.PORT_CODE --港ごと集計

),

SYUKKOKU AS(
SELECT
I.PORT_CODE  AS "港コード",
P.PORT_NAME   AS  "港名",
SUM(I.AMT)   AS  "出国者数"
--足りなければ足す
--↓JOIN
FROM IMMIGRATION AS I
INNER JOIN PORT AS P
  ON I.PORT_CODE = P.PORT_CODE
  
INNER JOIN GRP AS G
  ON I.GROUP_CODE = G.GROUP_CODE
  
WHERE
I.GROUP_CODE = '120' --しゅっこく者だけ

GROUP BY I.PORT_CODE --港ごと集計

)

SELECT 
N.港コード  AS "港コード",
N.港名   AS  "港名",
SUM(N.入国者数)   AS  "入国者数",
SUM(S.出国者数)   AS  "出国者数",
N.入国者数 - S.出国者数   AS  "差分"
FROM NYUKOKU AS N
LEFT OUTER JOIN SYUKKOKU AS S
  ON N.港コード = S.港コード
  
WHERE --入国者が出国者より多い
N.入国者数 > S.出国者数