Using Window functions for cumulatives

As a second example, consider the case where you're computing sales for each month orders were placed:

SELECT EXTRACT(year FROM orderdate) AS year, EXTRACT(month FROM orderdate) AS month,sum(netamount) as sales FROM orders GROUP BY year,month ORDER BY year,month;
 year | month |   sales   
------+-------+-----------
 2004 |     1 | 199444.50
 2004 |     2 | 200269.86
 2004 |     3 | 194516.68
 2004 |     4 | 200131.18
 2004 |     5 | 197131.77
 2004 |     6 | 199919.21
 2004 |     7 | 190953.70
 2004 |     8 | 194102.55
 2004 |     9 | 196798.83
 2004 |    10 | 195798.80
 2004 |    11 | 199882.86
 2004 |    12 | 202769.80

What if you wanted a cumulative total at each point during the year? As that problem also requires referencing things that are in other rows, it's again not something you can do in the standard SQL model easily. Try it out; it's another fun challenge. Possible solutions include building a complicated query that...