That's all we wanted to teach you in this part. It's time to wrap things up! First, let's review what we've learned:

1. To show the total revenue for each year, use DATEPART():

   SELECT
     DATEPART(Year, OrderDate) AS RevenueYear,
     SUM(Amount) AS TotalRevenue 
   FROM Orders
   GROUP BY DATEPART(Year, OrderDate)
   ORDER BY DATEPART(Year, OrderDate);
   

2. To show the total revenue for each quarter/month in each year, use DATEPART() twice:

   SELECT
     DATEPART(Year, OrderDate) AS RevenueYear, 
     DATEPART(Quarter, OrderDate) AS RevenueQuarter,
     ...
   GROUP BY
     DATEPART(Year, OrderDate),
     DATEPART(Quarter, OrderDate);
   

3. To show the total revenue on all aggregation levels, add ROLLUP():

   ...
   GROUP BY ROLLUP(
     DATEPART(Year, OrderDate),
     DATEPART(Quarter, OrderDate)
   )
   ...
   

4. To show the revenue from the previous year, use LAG():

   LAG(SUM(Amount), 1) OVER (ORDER BY DATEPART(Year, OrderDate))
   

5. To calculate the difference between the current period and the previous period, use:

   SUM(Amount) - LAG(SUM(Amount), 1) OVER (ORDER BY DATEPART(Year, OrderDate)) AS Delta
   

6. To calculate the difference between the current period and the previous period as a percentage, use:

   ROUND(100 * (
     SUM(Amount) - LAG(SUM(Amount), 1) OVER (ORDER BY DATEPART(Year, OrderDate)) /
     CAST(LAG(SUM(Amount), 1) OVER (ORDER BY DATEPART(Year, OrderDate)) AS float)),
     2) AS DeltaPercentage
   

7. To create a revenue in quarters report, use:

   SELECT
     DATEPART(Year, OrderDate) AS Year, 
     SUM(CASE WHEN DATEPART(Quarter, OrderDate) = 1 THEN Amount ELSE 0 END) AS Q1,
     ...
   FROM Orders
   GROUP BY DATEPART(Year, OrderDate)
   ORDER BY DATEPART(Year, OrderDate);
   

How about a quick quiz?