Write a query to return the listing of customers who did not make purchase during the invoicing period. Sort the results by customer code. 2. [4 marks] Write a query to list the most popular product's p_code and p_descript. Product's popularity is defined by the total quantity that have already been sold [Hint: you can use view] 3. [4 marks] Write a PL/SQL anonymous block to list those customers' full name who has some balance. [Hint: use cursor] 4. [3 marks] Write a PL/SQL anonymous block to calculate the subtotal of an invoice. Initialize a scalar variable to a specified invoice.