Create calculated field based on previous rows Oracle SQL - sql

Question

I am trying to create a manually calculated column where I keep track of a current inventory.
Currently, I have a table that looks like this:
| Group | Part | Operation Type | Transaction Amount |
|--------------|------|----------------|--------------------|
| Concrete | A | STOCK | 100 |
| Concrete | A | Buy | 25 |
| Concrete | A | Make | -10 |
| Concrete | A | Make | -10 |
| Concrete | A | Make | -10 |
| Concrete | A | Make | -10 |
| Concrete | A | Make | -10 |
| Concrete | B | STOCK | -10 |
| Concrete | B | Make | -10 |
| Concrete | B | Make | -10 |
| Concrete | B | Make | -10 |
| Concrete | B | Make | -10 |
| Concrete | B | Make | 150 |
| Construction | C | STOCK | 10 |
| Construction | C | Make | -1 |
| Construction | C | Make | -1 |
| Construction | C | Make | -1 |
| Construction | C | Make | -1 |
| Construction | D | STOCK | 5 |
| Construction | D | Make | -5 |
The table is first ordered by group then by part, and then STOCK is always shown as the first value. The idea is to create a new manually calculated column, curr_inventory, that allows for us to keep track of current inventory and see if or when our inventory for a given part, for a given group, dips below 0.
Ideally, the end results would look like this:
| Group | Part | Operation Type | Transaction Amount | New_Inventory_Column |
|:------------:|:----:|:--------------:|:------------------:|:--------------------:|
| Concrete | A | STOCK | 100 | 100 |
| Concrete | A | Buy | 25 | 125 |
| Concrete | A | Make | -10 | 115 |
| Concrete | A | Make | -10 | 105 |
| Concrete | A | Make | -10 | 95 |
| Concrete | A | Make | -10 | 85 |
| Concrete | A | Make | -10 | 75 |
| Concrete | B | STOCK | 10 | 10 |
| Concrete | B | Make | -10 | 0 |
| Concrete | B | Make | -10 | -10 |
| Concrete | B | Make | -10 | -20 |
| Concrete | B | Make | -10 | -30 |
| Concrete | B | Make | 150 | 120 |
| Construction | C | STOCK | 10 | 10 |
| Construction | C | Make | -1 | 9 |
| Construction | C | Make | -1 | 8 |
| Construction | C | Make | -1 | 7 |
| Construction | C | Make | -1 | 6 |
| Construction | D | STOCK | 5 | 5 |
| Construction | D | Make | -5 | 0 |
The end result would be a column that initiates when the part number has changed and the operation type is STOCK, and then begins to calculate (using the transaction amount) what the current inventory is.
I am not sure where to start on a SQL query that would allow for this. Intuitively, the pseudocode would look something like:
for each row in table:
if operation_type == "stock":
curr_inv = stock.value
else:
curr_inv = previous_curr_inv + transaction_amount
However, I am not sure how to even begin writing SQL for this. I typically try to post what SQL I am working with but I don't even know where to begin. I have looked at various posts online, on SO, including posts like this, and this, and this, but I could not see how the selected answers could be used as a solution.

Answer 1

I used the window function to calculate the running total.
I added the row_number column in the subquery.
Try this:
select t1."Group",t1."Part",t1."Operation Type", t1."Transaction Amount",
sum(t1."Transaction Amount") over (partition by t1."Group",t1."Part" order by t1.rownumber)
from (
select row_number() over (order by null) as rownumber, t.*
from test t ) t1
Test Result:
DB<>Fiddle

Answer 2

SQL tables represent unordered sets. To put things in order, you need a column with the ordering. In the code below, I will use ? for this column.
Then you want a cumulative sum:
select t.*,
sum(case when operation_type in ('STOCK'),
when operation_type in ('Make') then - amount
when operation_type in ('Buy') then - amount
else 0
end) over (partition by group, part order by ?)
from t;