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

Edit: I missed the point that it should average the three previous records...

For a general running average, I think something like this would work:

SELECT
    id, number, 
    SUM(number) OVER (ORDER BY ID) / 
       ROW_NUMBER() OVER (ORDER BY ID) AS [RunningAverage]
FROM myTable
ORDER BY ID