Getting invalid column name when creating a procedure using multiple if statements

Question

If someone could point me in the right direction. I have a homework problem where I need to create a stored procedure that takes in a product name and will check how much a customer is spending. Depending on how much is spent a discount increase will be added to the existing discount.

I've been referencing the videos and SQL scripts provided by the teacher. I've also checked w3schools, sqlshack, and a few other websites to finish the project and I've reached a point where I'm stuck. Any of the column names in the if statement gives me an error message of "Invalid Column Name". If I add the table name to the column, I get a different error message of "The multi-part identifier could not be bound". I have moved the inner joins to inside the if statements and didn't change anything.

    
Use Northwind;

go

CREATE PROCEDURE CategoryDiscount
 @categoryname varchar(50)

 AS

Select [Order Details].UnitPrice, Quantity from [Order Details] 
INNER JOIN Products on [Order Details].ProductID= Products.ProductID
INNER JOIN Categories on Categories.CategoryID = [Order Details].ProductID

If (UnitPrice * Quantity) < 50
    begin
        update [Order Details] 
        set Discount = Discount + .05
        Where CategoryName = '@categoryname';
    end
ELSE IF (UnitPrice * Quantity) between 50 and 100
    begin
        update [Order Details] 
        set Discount = Discount + .10
        Where CategoryName = '@categoryname';
    end
ELSE IF (UnitPrice * Quantity) > 100
    begin
        update [Order Details] 
        set Discount = Discount + .15
        Where CategoryName = '@categoryname';
    end
Else
    begin
        print 'Incorrect'
    end;

Answer 1

You should store values UnitPrice and Quantity in the variables, those could be @UnitPrice and @Quantity and it will be in the select statement which comes in the start of your stored procedure. Then use those variables in your if statements.