Help !!! - partition by clause

Question

hi. i have the following table
table : item_tracker
date | item_code | begin_price
end of each day, one record for each item is getting written to this table. now after the records are written, i want to calculate the change in begin prices
the formula is change_in_price=(todays_begin / yesterdays_begin) * 100
i wrote the following query but it seems not working. please advise me on what i am doing wrong.. also is the approach wrong ?
please advise.
select date, item_code, begin_price, first_value(begin_price) over (partition by date, item_code order by trunc(date) desc rows between 2 preceding and 1 preceding) as yesterdays_begin
from item_tracker
here, the partition by column is not showing any value.
is there any way i can include the date ? (like a 'having clause' for group by, what is the method to partition by)
i tried range between as well. it reurned the error that i need to mention a number value instead of date to check a range.
please help me :(
thanx in advance

Answer 1

Yes, joins are effective :
SQL> create table item_tracker(price_date date, item_id number, day_price number);
Table created.
SQL> insert into item_tracker values ('11-JAN-11',1,10);
1 row created.
SQL> insert into item_tracker values ('11-JAN-11',2,12);
1 row created.
SQL> insert into item_tracker values ('11-JAN-11',3,24);
1 row created.
SQL> insert into item_tracker values ('12-JAN-11',1,10.5);
1 row created.
SQL> insert into item_tracker values ('12-JAN-11',2,16);
1 row created.
SQL> insert into item_tracker values ('12-JAN-11',3,21);
1 row created.
SQL> commit;
Commit complete.
SQL>
SQL> l
  1  select a.item_id, a.price_date, a.day_price, b.day_price PrevPrice, (a.day_price - b.day_price) PriceDiff
  2  from item_tracker a, item_tracker b
  3  where a.item_id=b.item_id
  4  and a.price_date=to_date('12-JAN-11','DD-MON-RR')
  5  and b.price_date=a.price_date-1
  6* order by 1
SQL> /
   ITEM_ID PRICE_DAT  DAY_PRICE  PREVPRICE  PRICEDIFF
         1 12-JAN-11       10.5         10         .5
         2 12-JAN-11         16         12          4
         3 12-JAN-11         21         24         -3
SQL>Hemant K Chitale