In this example, AVG is used as a window function to compute the moving average of all product sales, by month, in the year 2000. Note that the WINDOW specification uses a RANGE clause, which causes the window bounds to be computed based on the month value, and not by the number of adjacent rows as with the ROWS clause. Using ROWS would yield different results if, for example, there were no sales of some or all the products in a particular month.

SELECT *
  FROM ( SELECT s.ProductID, 
         Month( o.OrderDate ) AS julian_month,
         SUM( s.Quantity ) AS sales,
         AVG( SUM( s.Quantity ) )
         OVER ( PARTITION BY s.ProductID
           ORDER BY Month( o.OrderDate ) ASC
           RANGE BETWEEN 1 PRECEDING AND 1 FOLLOWING ) 
         AS average_sales
         FROM SalesOrderItems s KEY JOIN SalesOrders o
         WHERE Year( o.OrderDate ) = 2000
         GROUP BY s.ProductID, Month( o.OrderDate ) ) 
  AS DT
  ORDER BY 1,2;

 See also