MySQL: Group by two columns and sum

Building an inventory system. I have lots of products and each product has three different variables. So for stock totals I want to group by the two columns (product & size) and sum quantity to get stock total.

product	Size	Quantity
Widget one	2	275
Widget one	2	100
Widget two	3	150
Widget two	2	150

What I want for output:

product	Size	Quantity
Widget one	2	375
Widget two	3	150
Widget two	2	150

I figured out how to group by one column and sum using the code below:

$query = "SELECT product, SUM(Quantity) FROM inventory GROUP BY product";  
$result = mysql_query($query) or die(mysql_error());
// Print out result
while($row = mysql_fetch_array($result)){
    echo "Total ". $row['product']. " = ". $row['SUM(Quantity)'];
    echo "<br />";
}
?>

I am just stuck on grouping by both columns. Is it possible? or should I just create three different products for the of the three sizes and eliminate that column? Thanks.

Answer

Based on your example table, it appears you want to be grouping on product rather than id. You merely need to add the Size column to both the SELECT list and the GROUP BY

$query = "SELECT 
            product,
            Size, 
            SUM(Quantity) AS TotalQuantity 
          FROM inventory
          GROUP BY product, Size";

Note that I have added a column alias TotalQuantity, which will allow you to more easily retrieve the column from the fetched row via the more sensible $row['TotalQuantity'], rather than $row['SUM(Quantity)']