ORACLE: Updating multiple columns at once

I guess the issue here is that you are updating INV_DISCOUNT and the INV_TOTAL uses the INV_DISCOUNT. so that is the issue here. You can use returning clause of update statement to use the new INV_DISCOUNT and use it to update INV_TOTAL.

this is a generic example let me know if this explains the point i mentioned

CREATE OR REPLACE PROCEDURE SingleRowUpdateReturn
IS
    empName VARCHAR2(50);
    empSalary NUMBER(7,2);      
BEGIN
    UPDATE emp
    SET sal = sal + 1000
    WHERE empno = 7499
    RETURNING ename, sal
    INTO empName, empSalary;

    DBMS_OUTPUT.put_line('Name of Employee: ' || empName);
    DBMS_OUTPUT.put_line('New Salary: ' || empSalary);
END;

It's perfectly possible to update multiple columns in the same statement, and in fact your code is doing it. So why does it seem that "INV_TOTAL is not updating, only the inv_discount"?

Because you're updating INV_TOTAL with INV_DISCOUNT, and the database is going to use the existing value of INV_DISCOUNT and not the one you change it to. So I'm afraid what you need to do is this:

UPDATE INVOICE
   SET INV_DISCOUNT = DISC1 * INV_SUBTOTAL
     , INV_TOTAL    = INV_SUBTOTAL - (DISC1 * INV_SUBTOTAL)     
WHERE INV_ID = I_INV_ID;

        

Perhaps that seems a bit clunky to you. It is, but the problem lies in your data model. Storing derivable values in the table, rather than deriving when needed, rarely leads to elegant SQL.