Good job! Now that we know how to add TotalRevenue 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 AllCategories AS (
  SELECT SUM(Amount) AS TotalSum
  FROM Orders
)
SELECT 
  CategoryName,
  SUM(OI.Amount) AS CategoryRevenue,
  SUM(OI.Amount) / CAST(AC.TotalSum AS decimal(10,2)) AS TotalRevenueRatio
FROM AllCategories AC, OrderItems OI
JOIN Orders O
  ON O.OrderId = OI.OrderId
JOIN Products P
  ON OI.ProductId = P.ProductId
JOIN Categories C
  ON P.CategoryId = C.CategoryId
GROUP BY CategoryName, AC.TotalSum;

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.