Challenged: Big query question

Question

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

Answer 1

Is it this you were looking for
select id, status, start_date, end_date, lag_start_date
from (
select id,
status,
start_date,
decode(status, 'new', null, end_date) end_date,
lag(start_date) over (order by start_date) lag_start_date
from test)
where lag_start_date <= start_date ;
ID STA START_DAT END_DATE LAG_START
1 new 16-Feb-07 03-Feb-07
1 old 10-Mar-07 12-Mar-07 16-Feb-07
1 old 14-Mar-07 16-Mar-07 10-Mar-07