Getting previous value from previous row

Question

Hi,
My desired output should be
ID     Book_date     Credit     Previous_Credit
123IT1     12/8/2008     100800     72500
from this table:
ID     Book date     Credit
123IT1     12/8/2008     100800
123IT1     12/18/2007     72500
123IT1     11/14/2006     150720
I tried to use the lead function but I'm having problem when I included the date range condtion:
This is my query:
select id, book_date, credit, lead(credit) over (order by rownum) as previous_credit
from t
where book_date between '01-December-2008' and '31-December-2008'
Result
ID     Book_date     Credit     Previous_Credit
123IT1     12/8/2008     100800     
The previous_credit column is always blank. I think because of the date range..?
Any idea how to get the right result? pls. help

Answer 1

Your expected output indicates that you may require a LAG
SQL> with t as
  2  (
  3    select '123IT1' id, to_date('12/08/2008','mm/dd/yyyy') Book_Date, 100800 Credit from dual uni
on
  4    select '123IT1', to_date('12/18/2007','mm/dd/yyyy'), 72500 from dual union
  5    select '123IT1', to_date('11/14/2006','mm/dd/yyyy'), 150720 from dual
  6  )
  7  --
  8  select id
  9        ,Book_Date
10        ,Credit
11        ,lag(Credit) over (partition by id order by Book_Date) Previous_Credit
12  from t
13  /
ID     BOOK_DATE     CREDIT PREVIOUS_CREDIT
123IT1 14-NOV-06     150720
123IT1 18-DEC-07      72500          150720
123IT1 08-DEC-08     100800           72500If you always require a Previous Credit then you may want to modify this:
(partition by id order by Book_Date)to this:
(order by id, Book_Date)