How to construct price time series from price changes using MS-Access 2007 queries? - ms-access

Question

Background
I've got a table of price changes tblPriceChanges of various items tblItems (example below - data made up):
[tblPriceChanges]:
Timestamp Item Price
9AM 01/01/2013 Orange 50p
9AM 01/01/2013 Apple 30p
2PM 01/01/2013 Pen 80p
2PM 02/01/2013 Orange 55p
2PM 02/01/2013 Pen 85p
9AM 03/01/2013 Apple 25p
9AM 05/01/2013 Pencil 10p
9AM 05/01/2013 Pen 70p
2PM 05/01/2013 Pencil 15p <- Notice there can be multiple price changes on the same day
...
[tblItems]:
Item Category Ratio
Orange Fruit 1
Apple Fruit 3
Pen Stationary 2
Pencil Stationary 5
...
Problem
The end result is that I want to be able to see how the average price of each category changes through time - for example:
Specifically, the average price series of Fruits, for instance, should be calculated as the weighted average of Orange and Apple prices in a Ratio of 1:3. So in the end I'm looking to generate (via some combination of queries) the following table for the underlying data of the chart:
Timestamp Fruit Stationary
01/01/2013 40.0 80.0
02/01/2013 55.0 85.0
03/01/2013 50.0 85.0
04/01/2013 50.0 85.0
05/01/2013 50.0 75.0
...
(this data is also made up and so probably not consistent with the original example)
I've managed to get an inner join on the two tables, but not too sure how to proceed. My main problem is how to handle days with no prices changes such as 01/04/13. The average prices still exist, but is not getting picked up by any query I try.
So how to use queries to construct data for the chart?

Answer 1

One remark before we start on a solution: be careful not to use reserved words for field names. Timestamp is a reserved word. Access will let you name a column that way, but you may encounter strange issues later on, especially if you ever move to another database in the future or use some other tools that fetch data from your Access database.
So here, I renamed your Timestamp column to DateTimeStamp.
I think that your requirements would be more straightforward to implement in VBA rather than in pure SQL queries: while you can easily build a query that gets you the average of each category per day, you are going to struggle to fill-in the data for the days where you have no data.
Simple but incomplete SQL solution
However, maybe having these holes in your data isn't that much of an issue since the graph will simply skip those missing values (it's not like they would show up as a data value of 0.00).
In that case, the following query should give you the results:
SELECT Dateserial(Year([DateTimeStamp]),
Month([DateTimeStamp]),
Day([DateTimeStamp])) AS NormalisedDate,
tblItems.Category,
SUM([Price]*[Ratio])/SUM([Ratio]) AS AvgPrice
FROM tblPriceChanges
INNER JOIN tblItems
ON tblPriceChanges.Item=tblItems.Item
GROUP BY Dateserial(Year([DateTimeStamp]),
Month([DateTimeStamp]),
Day([DateTimeStamp])),
tblItems.Category
Would result in the following:
NormalisedDate Category AvgPrice
01/01/2013 Fruit 35
01/01/2013 Stationary 80
02/01/2013 Fruit 55
02/01/2013 Stationary 85
03/01/2013 Fruit 25
05/01/2013 Stationary 22.0833333333333