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 the n previous rows in the resultset.

Let me explain

Id        Number       Average
 1             1          NULL
 2             3          NULL
 3             2          NULL
 4             4             2 <----- Average of (1, 3, 2),Numbers from previous 3 rows
 5             6             3 <----- Average of (3, 2, 4),Numbers from previous 3 rows
 .             .             .
 .             .             .

The first 3 rows of the Average column are null because there are no previous rows. The row 4 in the Average column shows the average of the Number column from the previous 3 rows.

I need some help trying to construct a SQL Select statement that will do this.

Answer 1

This should do it:

--Test Data
CREATE TABLE    RowsToAverage
    (
    ID int NOT NULL,
    Number int NOT NULL
    )

INSERT  RowsToAverage(ID, Number)
SELECT  1, 1
UNION ALL
SELECT  2, 3
UNION ALL
SELECT  3, 2
UNION ALL
SELECT  4, 4
UNION ALL
SELECT  5, 6
UNION ALL
SELECT  6, 8
UNION ALL
SELECT  7, 10

--The query
;WITH   NumberedRows
AS
(
SELECT  rta.*, row_number() OVER (ORDER BY rta.ID ASC) AS RowNumber
FROM    RowsToAverage rta
)

SELECT  nr.ID, nr.Number,
        CASE
            WHEN nr.RowNumber <=3 THEN NULL
            ELSE (  SELECT  avg(Number) 
                    FROM    NumberedRows 
                    WHERE   RowNumber < nr.RowNumber
                    AND     RowNumber >= nr.RowNumber - 3
                )
        END AS MovingAverage
FROM    NumberedRows nr

Answer 2

Assuming that the Id column is sequential, here's a simplified query for a table named "MyTable":

SELECT 
    b.Id,
    b.Number,
    (
      SELECT 
       AVG(a.Number) 
      FROM 
       MyTable a 
     WHERE 
       a.id >= (b.Id - 3) 
       AND a.id < b.Id
       AND b.Id > 3 
     ) as Average
FROM 
    MyTable b;