$query = 'SELECT Sum(db_cart_product.cart_product_quantity*db_price.price_value) AS PRZ, db_category.min_sum
FROM db_cart INNER JOIN (((db_cart_product INNER JOIN db_price ON db_cart_product.product_id = db_price.price_product_id)
INNER JOIN db_product_category ON db_cart_product.product_id = db_product_category.product_id)
INNER JOIN db_category ON db_product_category.category_id = db_category.category_id) ON db_cart.cart_id = db_cart_product.cart_id
WHERE (((db_cart.session_id)="'.$session_id.'"))
GROUP BY db_category.category_id, db_category.min_sum;
';
	
	$result = mysql_query($query) or die("Query failed : " . mysql_error());

	
	
	    while ($line = mysql_fetch_array($result, MYSQL_ASSOC)) {
        print "\t<tr>\n";
        foreach ($line as $col_value) {
            print "\t\t<td class=\"chekout_price_table\">".ceil($col_value)."</td>\n";
        }
        print "\t</tr>\n";
    }

$row = mysql_fetch_row($result);
echo $row[0];
echo $row[1];

$col2_sum = 0; // тут сумма значений второго столбца
while ($row = mysql_fetch_array($result, MYSQL_ASSOC))
{
	if ($row['min_sum'] > $row['PRZ']) 
	{
		// ошибка
	}
	$col2_sum += $row['min_sum'];
}