Moving average query MS Access - ms-access

Question

I am trying to calculate the moving average of my data. I have googled and found many examples on this site and others but am still stumped. I need to calculate the average of the previous 5 flow for the record selected for the specific product.
My Table looks like the following:
TMDT Prod Flow
8/21/2017 12:01:00 AM A 100
8/20/2017 11:30:45 PM A 150
8/20/2017 10:00:15 PM A 200
8/19/2017 5:00:00 AM B 600
8/17/2017 12:00:00 AM A 300
8/16/2017 11:00:00 AM A 200
8/15/2017 10:00:31 AM A 50
I have been trying the following query:
SELECT b.TMDT, b.Flow, (SELECT AVG(Flow) as MovingAVG
FROM(SELECT TOP 5 *
FROM [mytable] a
WHERE Prod="A" AND [a.TMDT]< b.TMDT
ORDER BY a.TMDT DESC))
FROM mytable AS b;
When I try to run this query I get an input prompt for b.TMDT. Why is b.TMDT not being pulled from mytable?
Should I be using a different method altogether to calculate my moving averages?
I would like to add that I started with another method that works but is extremely slow. It runs fast enough for tables with 100 records or less. However, if the table has more than 100 records it feels like the query comes to a screeching halt.
Original method below.
I created two queries for each product code (There are 15 products): Q_ProdA_Rank and Q_ProdA_MovAvg
Q_ProdA_RanK (T_ProdA is a table with Product A's information):
SELECT a.TMDT, a.Flow, (Select count(*) from [T_ProdA]
where TMDT<=a.TMDT) AS Rank
FROM [T_ProdA] AS a
ORDER BY a.TMDT DESC;
Q_ProdA_MovAvg
SELECT b.TMDT, b.Flow, Round((Select sum(Flow) from [Q_PRodA_Rank] where
Rank between b.Rank-1 and (b.Rank-5))/IIf([Rank]<5,Rank-1,5),0) AS
MovingAvg
FROM [Q_ProdA_Rank] AS b;

Answer 1

The problem is that you're using a nested subquery, and as far as I know (can't find the right site for the documentation at the moment), variable scope in subqueries is limited to the direct parent of the subquery. This means that for your nested query, b.TMDT is outside of the variable scope.
Edit: As this is an interesting problem, and a properly-asked question, here is the full SQL answer. It's somewhat more complex than your try, but should run more efficiently
It contains a nested subquery that first lists the 5 previous flows for per TMDT and prod, then averages that, and then joins that in with the actual query.
SELECT A.TMDT, A.Prod, B.MovingAverage
FROM MyTable AS A LEFT JOIN (
SELECT JoinKeys.TMDT, JoinKeys.Prod, Avg(Top5.Flow) As MovingAverage
FROM (
SELECT JoinKeys.TMDT, JoinKeys.Prod, Top5.Flow
FROM MyTable As JoinKeys INNER JOIN MyTable AS Top5 ON JoinKeys.Prod = Top5.Prod
WHERE Top5.TMDT In (
SELECT TOP 5 A.TMDT FROM MyTable As A WHERE JoinKeys.Prod = A.Prod AND A.TMDT < JoinKeys.TMDT ORDER BY A.TMDT
)
)
GROUP BY JoinKeys.TMDT, JoinKeys.Prod
) AS B
ON A.Prod = B.JoinKeys.Prod AND A.TMDT = B.JoinKeys.TMDT
While in my previous version I advocated a VBA approach, this is probably more efficient, only more difficult to write and adjust.