Please help: Case Statement - ORA-01722: invalid number Error

Question

Hi All,
I'm trying to use Case statement to recode Cancelled statuses into two groups
'Reject' if the difference between first date and last date is less than 29 and
'Accept' if the difference between first date and last date is greater than 30
Here is my 'test' data:
STATUS     FIRST_DATE     LAST_DATE
Transfer     10/08/2011     10/09/2011
Mover     10/08/2011     15/09/2011
Cancel     10/08/2011     16/09/2011
Cancel     10/08/2011     5/09/2011
Here is the syntax
select a.*,
(CASE WHEN a.STATUS ='Cancel' THEN (round(a.LAST_DATE-a.FIRST_DATE))
ELSE 0
END) CAN_DAYS,
(CASE WHEN 'CAN_DAYS' >29 THEN 'Reject'
WHEN 'CAN_DAYS' <30 THEN 'Accept'
END) Reject_Accept
from test a
The first CASE statement works fine and gives me this
STATUS     FIRST_DATE     LAST_DATE     CAN_DAYS
Transfer     10/08/2011     10/09/2011     0
Mover     10/08/2011     15/09/2011     0
Cancel     10/08/2011     16/09/2011     37
Cancel     10/08/2011     5/09/2011     26
but the second CASE produces ORA-01722: invalid number Error. It is something to do with the CAN_DAYS data type, Oracle doesn't see it as Numeric i think.
The result i want to see would be
STATUS     FIRST_DATE     LAST_DATE     CAN_DAYS Reject_Accept
Transfer     10/08/2011     10/09/2011     0
Mover     10/08/2011     15/09/2011     0
Cancel     10/08/2011     16/09/2011     37 Accept
Cancel     10/08/2011     5/09/2011     26 Reject
Could someone please help me to fix the syntax?
Thank you in advance,

Answer 1

This seems to give you your desired results...
with test as
  ( select 'Transfer' as status, to_date('10/08/2011') as first_date, to_date('10/09/2011') as last_date  from dual union all
    select 'Mover'             , to_date('10/08/2011')              , to_date('15/09/2011')               from dual union all
    select 'Cancel'            , to_date('10/08/2011')              , to_date('16/09/2011')               from dual union all
    select 'Cancel'            , to_date('10/08/2011')              , to_date('05/09/2011')               from dual
select  status
     ,  first_date
     ,  last_date
     ,  CAN_DAYS
     ,  CASE
          WHEN CAN_DAYS =   0 THEN NULL
          WHEN CAN_DAYS <  30 THEN 'Reject'
          WHEN CAN_DAYS >= 30 THEN 'Accept'
        END  Reject_Accept
  from  ( select  status
               ,  first_date
               ,  last_date
               ,  CASE
                    WHEN STATUS ='Cancel' THEN round(LAST_DATE-FIRST_DATE)
                    ELSE 0
                  END  CAN_DAYS
            from  test
;