SELECT IM1.PORT_CODE AS '港コード'
     , por.PORT_NAME AS '港名'
     , IM1.amt AS '入国者数'
     , IM2.amt AS '出国者数'
     , IM1.amt - IM2.amt AS '差分'
         
 FROM IMMIGRATION AS IM1
 
 LEFT JOIN IMMIGRATION AS IM2
   ON IM1.PORT_CODE = IM2.PORT_CODE
  AND IM1.GROUP_CODE = IM2.GROUP_CODE
  AND IM2.KIND_CODE = '120'
  
 LEFT JOIN PORT AS  por
   ON IM1.PORT_CODE = por.PORT_CODE

WHERE IM1.KIND_CODE = '110'
  AND IM1.amt - IM2.amt > 0
ORDER BY IM1.amt - IM2.amt DESC, IM1.PORT_CODE ASC