USE OLAP
--We can also do running totals using order by
SELECT year,month,total_sales
       ,sum(total_sales) over(order by year,month) as yearly_total
FROM sales_by_year_and_month
ORDER BY year,month

--Different display order
SELECT year,month,total_sales
       ,sum(total_sales) over(order by year,month) as running_total
FROM sales_by_year_and_month
ORDER BY month

--In most cases this doesn't make sense, but we will see a situation later in the chapter
--where this is useful.  