如何从数据库中的值总和中减去值

Question

我应该从数据库中的值总和中减去给定值！

例如： 用户“John”在他的帐户中填写了 2 次 100 美元和 50 美元，他帐户中的总计 150 美元 当他以 50 美元买东西时，我们应该从他帐户中的总金额中减去！ 并且应该更新总金额。

这里显示他帐户中的总金额的代码：

 <?php
    session_start();
    $sql = "SELECT sum(payment_amount) as 'payment' from payments 
    WHERE username= '" . $_SESSION['username'] . "' ";
    $result = $db->query($sql);
    $row = $result->fetch_array(MYSQLI_ASSOC);
    echo "" . $row['payment'] . "";
    ?>

表格：付款

  ID      username    payment_amount    Status  
+-------+-------------+-------------+-----------+
|   1   |  John       |     100     | Complete  |
+-------+-------------+-------------+-----------+
|   2   |  John       |     50      | Complete  |
+-------+-------------+-------------+-----------+
|   3   |  Alex       |     100     | Complete  |
+-------+-------------+-------------+-----------+

Answer 1

您在使用余额时不应该这样做。

最好的方法是像这样创建一个新表。

ID |用户名 |余额

然后每次付款，您只需在用户余额中使用 +$amount 更新 DB，而且，如果他花钱，您只需在 UPDATE 中执行 -$amount。

例子：

tbl_user

ID |姓名 |电子邮件

1..alex...blabla@test.com

---

tbl_balance：

ID |用户名 |余额

1....1.....100

Answer 2

是的，我可以向您展示我将如何做到这一点，但它不是 100% 满的，而是向您展示它是如何工作的。

/////PAYMENT PAGE/////

<?php 
/////Get the ID of user using his session username and after all you query your     user table WHERE username = $_SESSION["username"] and then you take the ID 
////And you save the id of user in a variable 
$getId = $row["id"];

/////Get the total balance of user right now using his id and save in variable
$getAmountNow = $row["balance"];
?>

<form action="myscript.php" method="get"> 
How much you want to pay <input type="text" name="amountPaid"><br>
<input type="hidden" name="<?php echo $getId; ?>"><br>
<input type="hidden" name="<?php echo $getAmountNow; ?>"><br>
<input type="submit" value="Submit">
</form> 
/////PAYMENT PAGE/////



/////myscript.php/////

$idUser = htmlspecialchars($_POST["id"]);
$amountNowUser = htmlspecialchars($_POST["balance"]);
$amountPaid = htmlspecialchars($_POST["amountPaid"]);

////If this is a spent
$newAmount = $amountNowUser - $amountPaid;

////If this is a buy
$newAmount = $amountNowUser + $amountPaid;

// prepare and bind$stmt = $conn->prepare("UPDATE tbl_balance SET balance=? WHERE     idUser=?");
$stmt->bind_param("ss", $newAmount, $idUser);
$stmt->execute();


/////myscript.php/////