Good! Let's get started. There are two queries which we can use for total revenue as follows:

SELECT SUM(Amount) AS TotalRevenue FROM Orders;

Or:

SELECT SUM(Amount) AS TotalRevenue FROM OrderItems;

Now, let's say we want to show the history-to-date revenue for various categories:

Such a report can help us discover which product categories are more profitable. In this case, we can only use the second query (the one with the OrderItems table). Take a look:

SELECT 
  CategoryName,
  SUM(Amount) AS TotalRevenue 
FROM OrderItems OI
JOIN Products P
  ON OI.ProductId = P.ProductId
JOIN Categories C
  ON P.CategoryId = C.CategoryId
GROUP BY CategoryName;

This query is a bit more complicated because we had to join OrderItems with Products and Categories to get the category names used in the report.