MySQL - How do I Calculate an Average Value Over a Period of Time When Only Value Changes are Recorded

Question

I am tracking the sales price of various items, and I would like to calculate the average price of each item over a period of time.

My simplified tables look like this:

Items
------------------
|key  | title    |
------------------
|1    | Sproket 1|
|2    | Sproket 2|
------------------

And

Prices
----------------------------------------
|item_key | price | datetime           |
----------------------------------------
|1        | 2.99  | 2014-11-25 02:05:56|
|1        | 1.99  | 2014-12-13 02:05:56|
|2        | 9.99  | 2014-10-25 02:05:56|
|2        | 8.99  | 2014-11-13 02:05:56|
----------------------------------------

The Items table contains a single row for each item. The Prices table contains a single row for each price change. The script runs daily and checks for the current prices, if the current price is different than the most recent item price recorded in Prices than a new entry is made in Prices.

My question is, How do I calculate the average price of an item over 180 days?

I can't use AVG because there is not a price entry each day for each item.

Answer 1

Well, I started to write SQL to do the task. It got worse and worse. I needed several @variables and perhaps 3 levels of subqueries. So, I decided it was "impossible" in SQL, or at least very messy.

Instead, I would recommend either david's suggestion of having daily prices, which would make AVG work easily (probably no @variables or subqueries), or write it in some "real" programming language (PHP, VB, Java, ...).

One ugly was doing date arithmetic -- with bounds of 180 days ago and today. Walking through the data ASCENDING led to one problem; DESC lead to a different problem.

Answer 2

KISS

Assuming daily means every day including weekends and holidays.

1. Insert price for all items everyday
2. Use AVG() for value
3. Delete prices over 180 days.

Answer 3

Your probably going to have to use group_concat function to aggregate by key so you can do math on those groups of values.