Good job! Now that we know how to add total_revenue as the last column, we can calculate the ratio between a category's revenue and total revenue. Such a report can help us quickly identify which categories generate the most revenue. Here's the query:

WITH all_categories AS (
  SELECT SUM(amount) AS total_sum
  FROM orders
)
SELECT 
  category_name,
  SUM(oi.amount) AS category_revenue,
  SUM(oi.amount) / CAST(ac.total_sum AS DECIMAL(10,2)) AS total_revenue_ratio
FROM all_categories ac, order_items oi
JOIN orders o
  ON o.order_id = oi.order_id
JOIN products p
  ON oi.product_id = p.product_id
JOIN categories c
  ON p.category_id = c.category_id
GROUP BY 
  category_name,
  ac.total_sum;

In the third column, we now divide category revenue by total revenue and get the correct ratio. As usual, we're casting one of the values to a decimal type to avoid integer division.