Average Rating?

[Tim_L]

I've set up a successful online review system where users vote using a five star basis, amongst other things.

My question is: how would I be able to work out the average rating out of five and display it next to the product in the listing page? I know how to display a rating as a picture but I'm not sure how to do the calculation, if that's what it is?

Also, what happens when their isn't a review for the product?

[drawmack]

assuming you have a table reviews and each review has a product_id and a num_stars column you would do it like so

$sql = "SELECT COUNT(product_id) FROM reviews WHERE product_id = $_POST['product_id']";
$num_reviews = @mysql_result(@mysql_query($sql),0,0);

if($num_reviews > 0) {
    $sql = "SELECT SUM(num_stars) FROM reviews WHERE product_id = $_POST['product_id'] GROUP BY product_id";
    $tot_stars = @mysql_result(@mysql_query($sql),0,0):
    $avg_stars = round(($tot_stars/$num_reviews),2);
} else {
    $tot_stars = 0;
    $avg_stars = 0;
} //end if