SQL Server rolling average

Question

select
    Date, [Close],
    avg([close]) over (order by date asc rows 4 preceding) as MA10
from 
    dbo.APPL

This query returns the rolling 4 day average of stock prices. However I want to show the values as NULL if preceding days are fewer than 4.

How do I adjust that in SQL Server?

Answer 1

I think JeffUK meant doing something like this:

select
    Date, [Close],
    case when days >= 4 then MA10 end as MA10
from
(
    select
        Date, [Close],
        avg([close]) over (order by date asc rows 4 preceding) as MA10,
        count([close]) over (order by date asc rows 4 preceding) as days
    from 
        dbo.APPL

) X