php - 如何告诉 php 计算数据并从数据库中 +/- 它以获得新数字

Question

我一直在做库存管理项目，一切都很好，直到我遇到这种情况，我需要告诉 PHP 从数据库中存在的数量中减去行中给出的数量。但总是出问题并阻止脚本工作。

如果我有 20 件 T 恤，我订购了 2 件，那么 php 会更新数据库并将数量设置为 18。

这是我的代码:

<?php   
    require_once 'connections/dbc.php';
    $user_name= mysqli_real_escape_string($conn, $_POST['user_name']);
    $brand_name= mysqli_real_escape_string($conn, $_POST['brand_name']);
    $product_name= mysqli_real_escape_string($conn, $_POST['product_name']);
    $amount = mysqli_real_escape_string($conn, $_POST['amount']);
    $discount= mysqli_real_escape_string($conn, $_POST['discount']);
    $total_preis= mysqli_real_escape_string($conn, $_POST['total_preis']);
    $payment_type= mysqli_real_escape_string($conn, $_POST['payment_type']);

    if(isset($_POST['orderbutton'])):
      $sql = "INSERT INTO `orders` (`order_id`, `order_date`, `user_name`, `brand_name`, `product_name`, `amount`, `discount`, `total_preis`, `payment_type`) VALUES ('', 'NOW()','$user_name','$brand_name','$product_name','$amount','$discount', '$total_preis', '$payment_type')";

    for($x = 0; $x < count($_POST['product_name']); $x++) {     
        $updateProductAmountSql = "SELECT product.amount FROM product WHERE product.product_name = ".$_POST['product_name'][$x]."";
        $updateProductAmountData = $connect->query($updateProductAmountSql);

        while ($updateProductAmountResult = $updateProductAmountData->fetch_row()) {
          $updateAmount[$x] = $updateProductAmountResult[0] - $_POST['amount'][$x];             
            // update product table
            $updateProductTable = "UPDATE product SET amount = '".$updateAmount[$x]."' WHERE product_name = ".$_POST['product_name'][$x]."";
            $connect = query($updateProductTable);
        } // while  
    }// /for amount
    endif;
?>

Answer 1

你的查询和周围的代码是......好吧，有点乱<_>

去掉你的For/while，你只需要将订购的值(value)减去库存即可:

$sql2 = "UPDATE product 
    SET amount = amount - ".$amount." 
    WHERE product_name = '".$product_name."'";