如何比较两列并在Oracle中将最大的列作为新列返回

Question

Here is what I've tried:

SELECT
  cln.CLNID,
  MAX(cln.lastinputdate) clnlast ,
  MAX(subac.lastinputdate) sublast , 
  (CASE
      WHEN clnlast > sublast   
  THEN clnlast
      ELSE sublast 
  END) as lasttime
FROM
  IBROKER.cln cln
LEFT JOIN IBROKER.ClnSrvSubAc subac
ON
  cln.clnid=subac.clnid
GROUP BY
  cln.clnid,

But it ends up with a error "ORA-00904: "CLNLAST": invalid identifier"

Please help.

Answer 1

You can't reference an alias defined in your select list elsewhere in the select list. You'd need to use the actual column names. You also don't need to use a case here-- Oracle provides a greatest function

 greatest( max( cln.lastinputdate ), max( subac.lastinputdate ) )

Answer 2

You can not use column alias in this case. I think it should be:

SELECT
  cln.CLNID,
  MAX(cln.lastinputdate) clnlast ,
  MAX(subac.lastinputdate) sublast , 
  (CASE
      WHEN MAX(cln.lastinputdate)> MAX(subac.lastinputdate)
  THEN MAX(cln.lastinputdate)
      ELSE MAX(subac.lastinputdate)
  END) as lasttime
FROM
  IBROKER.cln cln
LEFT JOIN IBROKER.ClnSrvSubAc subac
ON
  cln.clnid=subac.clnid
GROUP BY
  cln.clnid;