Select a count using the last entry of a table

Question

I have a huge project for which I am trying to learn some PL/SQL.
Context:
Entry gets put into the Orders table. What I want to do is create a trigger( out of scope for this question) that takes the last entry's product ID number and performs a count of all the PID's that match it. Then I want to compare that number with a number in the ProductTransaction table.
I.e.
Orders ProductTransaction
OID TID
PID PurchaseCount
o_TID Price
etc.....
Last entry that gets put in contains
OID PID TID
4 100 2
ProductTransaction
TID PurchaseCount Price
2 10 100.00
What I want to do is if the last entry into the Orders table was the OID 4 then I want to count all PID=100 and determine if the PurchaseCount has been reached yet.
The only thing I can think of is the following, but it doesn't ensure that the last PID is what we are counting:
declare
v_countpid number;
begin
select
count(pid) into v_countpid
from
orders
where OID = (select last(oid) from orders);
dc
Im thinking something like the above which populates the v_countpid
and then taking another variable which is a select into which takes the last TID entered and comparing them in an if statement of some sort
Let me know your thoughts on this....

Answer 1

They you both for your replies. I was pushing myself to stay up late last night and I guess I didn't included everything.
Here it goes.
The concept is to have someone buy a product lets say at 10 dollars and it will take 10 other people for you to finally get your product released to you.
So when I purhcase PRODUCT A and put the entry into the orders table it will be something like this:
OID = 1
PID = 101
TID = 2
TIMEOFPURCHASE=<timestamp>
etc........
Now once that is entered into the table I am going to have a trigger as an after insert that will count the total PID's in the table that match what I just put into it, as in this case it was PID=101. The count of PID=101 will then be matched against the ProductTransaction PurchaseCount field to determine if that product should be released to the person with the lowest timestamp.
So....
ProductTransaction
TID=2
PRICE=$10
PurchaseCount=10
ORDERS
OID PID TID TOP
1 101 2 3 am
2 101 2 5 am
3 30 1 12 pm
4 101 2 4 pm
Example. The last entry was OID=4 so it will count up the number of PID=101 and then that number is compared against ProductTransaction purchase count. If hasn't exceeded then nothing happens. If the number is matched then OID=1 is moved into the shipping table for processing.
Let me know if this still doesn't clarify, or if there is a better way to do this....