Please Help: Trouble with nested CASE statement and comparing dates

Question

Please tell me why the query below is always returning the bold null even when the start_date of OLD is greater than or equal to the start_date of NEW.
What I want to do is get the difference of the start_dates of two statuses ( Start_date of OLD - Start_date of NEW) if
1. end_date of NEW is not null
2. start_date of OLD is greater than start_date of NEW
else return null.
select id,
case when max(end_date) keep (dense_rank last order by decode(request_wflow_status,'New',1,0),start_date) is null then
null
else
          case when max(decode(status,'OLD',start_date,null)) > max(decode(status,'NEW',start_date,null))
          then max(decode(status,'OLD',start_date,null)) - max(decode(status,'NEW',start_date,null))
else
null
end
end result
from cc_request_status where id =1
group by id;

Answer 1

Avinash,
Thank you for your help.. Here is a more description of my problem..
Here is a sample of data I have for a table with four columns (id,status,start_date,end_date)
What I need to do is to get difference of the start dates of the maximum available dates, if data is valid. The pseducode is as follows:
IF end_date of New status is null
THEN return null
ELSE
IF start_date of old >= start_date of new
THEN return (start_date of old - start_date of new)
ELSE return null
I used the following query but always return the bold null
select id,
(case when max(end_date) keep (dense_rank last order by decode(status,'new',1,0),start_date) is null then
null
else
          (case when max(decode(status,'old',start_date,null)) >=
          max(decode(status,'new',start_date,null))
          then max(decode(status,'old',start_date,null)) - max(decode(status,'new',start_date,null))
else
null
end)
end) result
from tbl where id =1
Based on the below sample, I expected to get the following result; 14-Mar-07 - 16-Feb-07 which is the difference of the maximum start_dates of the two statuses. However the query is not working.. Please help me.. Thank you..
Id    Status    start_date      end_date
1     new      03-Feb-07      07-Feb-07
1     new      16-Feb-07      21-Feb-07
1     old      '10-Mar-07      12-Mar-07
1     old      '14-Mar-07      16-Mar-07