SELECT statement that answers this question: What is the total quantity purchased
for each product within each category? Return these columns:

category_name-The category_name column from the category table

name_of_product-The product_name column from the products table

quantity_purchased-The total quantity purchased for each product with orders in the Order_Items table

Use the WITH ROLLUP operator to include rows that givea summary for each category name as well as a row that gives the grand total. Use the IF and GROUPING functions to replace null values in the category_name and product_name columns with literal values if they're for summary rows.