How do I calculate a moving average using MySQL?

Question

I need to do something like:

SELECT value_column1 
FROM table1 
WHERE datetime_column1 >= '2009-01-01 00:00:00' 
ORDER BY datetime_column1;

Except in addition to value_column1, I also need to retrieve a moving average of the previous 20 values of value_column1.

Standard SQL is preferred, but I will use MySQL extensions if necessary.

Answer 1

This is just off the top of my head, and I'm on the way out the door, so it's untested. I also can't imagine that it would perform very well on any kind of large data set. I did confirm that it at least runs without an error though. :)

SELECT
     value_column1,
     (
     SELECT
          AVG(value_column1) AS moving_average
     FROM
          Table1 T2
     WHERE
          (
               SELECT
                    COUNT(*)
               FROM
                    Table1 T3
               WHERE
                    date_column1 BETWEEN T2.date_column1 AND T1.date_column1
          ) BETWEEN 1 AND 20
     )
FROM
     Table1 T1