Applying percent change based on unique identifier - sql

Question

I'm calculating margins from a sales table in SQL and I need to reduce sales amounts if there is a discount in a sale based on it's invoice number. For this sample a 25% discount is applied Invoice 123. The total sales amount for invoice 123 is $100.00 but there is a 25% discount applied to that amount. What I want to do is apply that 25% discount to all sales numbers for invoice 123 so I can get the actual revenue number.
Sample Data:
ID Type ProductType Amount
123 Sale Jeans 50.00
123 Sale T-Shirt 30.00
123 Sale Sock 20.00
123 Discount - 25% NULL -25.00
456 Sale Jeans 60.00
456 Sale T-Shirt 40.00
456 Sale Sock 70.00
Expected Result:
ID Type ProductType Amount Actual Amount
123 Sale Jeans 50.00 41.67
123 Sale T-Shirt 30.00 21.67
123 Sale Sock 20.00 11.67
123 Discount - 25% NULL -25.00 0.00
456 Sale Jeans 60.00 60.00
456 Sale T-Shirt 40.00 40.00
456 Sale Sock 70.00 70.00
I've tried creating a new column where I multiple the amount times the discount rate but I can't get the numbers correct because it needs to be applied to the invoice number each discount corresponds to.
I'd like to have a new column that shows the adjusted amount based on the discount rate and then the discount amount showing zero.

Answer 1

One way is to use window functions to calculate the sum of the negative values for an id divided by the number of non negative values for that id by CASE expressions as arguments to the functions. The result of the division can than be deducted from the price. Additionally use a CASE to get zero for the negative values.
SELECT t1.id,
t1.amount,
CASE
WHEN t1.amount <= 0 THEN
0
ELSE t1.amount
+ sum(CASE
WHEN t1.amount < 0 THEN
t1.amount
ELSE
0
END) OVER (PARTITION BY t1.id)
/
count(CASE
WHEN t1.amount >= 0 THEN
1
END) OVER (PARTITION BY t1.id)
END actual_amount
FROM elbat t1;
db<>fiddle