How do I work out average rating and add a new column to my MySQL table to record this

I have a MySQL table of product ID's which has columns which show how many times each of these products has recieved a 1, 2, 3, 4 or 5 star rating from my customers. I would like to insert into the average rating column, which is currently blank, a figure to indicate what the average rating for each product is.

How can I calculate what the average rating for each product ID is and then modify the table to insert this average rating into the average rating column? I would then ideally like to run this SQL statement daily using a php script and a cron job to recalculate the average rating and update this table as more ratings are recorded for each product.

An example of the layout of the table is below:

Product ID | 1 | 2 | 3 | 4 | 5 | Average Rating

1294518    | 0 | 3 | 3 | 2 | 4 | ?

9226582    | 0 | 0 | 1 | 0 | 0 | ?

3946583    | 0 | 3 | 6 | 1 | 0 | ?

7392588    | 1 | 0 | 0 | 0 | 0 | ?

1196585    | 0 | 1 | 0 | 5 | 6 | ?

4285385    | 0 | 2 | 3 | 3 | 1 | ?

Solution

Have you tried:

UPDATE table
SET aveRate = (rate1 + rate2*2  + rate3*3  + rate4*4 + rate5*5) /
                           (rate1 + rate2 + rate3 + rate4 + rate5)

Am I missing something?