hi all , i want to take out of the data (more than one table) the
total purchases and the total sales, put each in a column near to each
others and group the rows by months and years, so i thought in pseudo
code something like this would work:

if to_char of date (MM-YY).sales = to_char of date (MM-YY).purchase
and prod.code.sal = prod.cod.purch than qty*unit_price(x) in "tot
purch" and qty*unit_price(y)in "tot sales"
from tables
where prod_cod = prod_cod
so with this logic i did this:

select TO_CHAR(purinv_1.Inv_Date,'mm') || ' - ' ||
TO_CHAR(purinv_1.Inv_Date,'yy') as "MONTH",
sum(case when to_char(salpmt_1.pmt_date,'MM-yy') =
to_char(purpmt_1.pmt_date,'MM-yy') and to_char(salpmt_1.pmt_date,'mm-
YY') = to_char(purpmt_1.pmt_date,'mm-YY') then
(sales_1.QTY*prod_1.UNIT_PRICE)else 0 end) as "Total Sales",
sum(case when to_char(salpmt_1.pmt_date,'MM-yy') =
to_char(purpmt_1.pmt_date,'MM-yy')
and to_char(salpmt_1.pmt_date,'mm-YY') = to_char(purpmt_1.pmt_date,'mm-
YY') then (purch_1.QTY*purch_1.UNIT_PRICE)else 0 end) as "Total
Purchase"

from purinv_1, sales_1,purpmt_1,salpmt_1, prod_1,purch_1
where sales_1.prod_code = purch_1.prod_code
group by TO_CHAR(purinv_1.Inv_Date,'mm'),
TO_CHAR(purinv_1.Inv_Date,'yy')
order by TO_CHAR(purinv_1.Inv_Date,'yy')

eventhough i dont see anything wrong(**to my knowledge)
but i am having the following problem :
whenever i try to execute it the cpu goes 100% for about 5 -6 min and
than crashes what would the cause be ?