Write a SELECT statement that answers this question: What is the total quantity
purchased for each product within each category? Return these columns:
The category_name column from the category table
The product_name column from the products table
The total quantity purchased for each product with orders in the Order_Items
table
Use the WITH ROLLUP operator to include rows that give a 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.