Great work! We can also create another version of this report by changing revenue values into revenue deltas. The end result should look like this:

We can use the following query:

SELECT 
  DATEPART(Year, OrderDate) AS RevenueYear,
  SUM(OI.Amount) - LAG(SUM(OI.Amount)) OVER (ORDER BY DATEPART(Year, OrderDate)) AS TotalRevenueDelta, 
  SUM(CASE WHEN C.CategoryName = 'Beverages' THEN OI.Amount ELSE 0.0 END)
  - LAG(SUM(CASE WHEN C.CategoryName = 'Beverages' THEN OI.Amount ELSE 0.0 END)) OVER (ORDER BY DATEPART(Year, OrderDate)) AS BeveragesRevenueDelta,
  SUM(CASE WHEN C.CategoryName = 'Seafood' THEN OI.Amount ELSE 0.0 END)
  - LAG(SUM(CASE WHEN C.CategoryName = 'Seafood' THEN OI.Amount ELSE 0.0 END)) OVER (ORDER BY DATEPART(Year, OrderDate)) AS SeafoodRevenueDelta 
FROM OrderItems OI 
JOIN Orders O
  ON OI.OrderId = O.OrderId
JOIN Products P
  ON P.ProductId = OI.ProductId
JOIN Categories C
  ON C.CategoryId = P.CategoryId
GROUP BY DATEPART(Year, OrderDate)
ORDER BY DATEPART(Year, OrderDate);

We now calculate each column as the sum from the current year (SUM(OI.Amount)) minus the sum from the previous year (LAG(SUM(OI.Amount)) OVER (ORDER BY DATEPART(Year, OrderDate))). To that end, we used the LAG() OVER() pattern we learned in the previous part.