SQL Select Statement For Calculating A Running Average Column

Question

I am trying to have a running average column in the SELECT statement based on a column from the n previous rows in the same SELECT statement. The average I need is based on

Answer 1

If you want this to be truly performant, and arn't afraid to dig into a seldom-used area of SQL Server, you should look into writing a custom aggregate function. SQL Server 2005 and 2008 brought CLR integration to the table, including the ability to write user aggregate functions. A custom running total aggregate would be the most efficient way to calculate a running average like this, by far.