mysql表

如何输出所选product_id的product_price SUM？

当前输出为"1245"。它似乎工作，但它将"124"放在"5"前面我想要的结果是"5"。

有关详细信息，请参见图像。

$query = "SELECT * FROM product_list WHERE school_subject_product_id = '1'"; //<---- There are 4 products under this id

$result = $mysqli->query($query);
    while($row = $result->fetch_assoc()){
        $product_id = $row["product_id"];
        $sql = "SELECT SUM(product_price) as total FROM products WHERE product_id = $product_id";
        $query = $mysqli->query($sql);
        $row = $query->fetch_assoc();
        $sum += $row['total'];
        echo $sum;
    }// end of while