How to insert the result in the desired row? SQL Server

I need the result that I got to be inserted in the right row.

DECLARE @DiscPrice float;
SET @DiscPrice = (SELECT Prod.priceProd - Prod.priceProd / 100 * Prod.disc 
                  FROM Prod 
                  WHERE id_prod = 1);

UPDATE Prod 
SET priceDisc = @DiscPrice 
WHERE id_prod = 1;

SELECT * FROM Prod;

That is, instead of WHERE id_prod = 1, there was something that inserted the desired result in all rows.

I’m not sure I made myself clear, but I hope that you will understand.

Answer

I think you want

UPDATE Prod 
SET priceDisc = ((priceProd - priceProd) / 100) * disc
WHERE id_prod = 1;

There is no need to use a variable or a query to assign the value to it.