我想从字段(B)中减去字段(A) - coin_charge值，并将结果存储在字段(B)中。

Question

首先，我在MySQL数据库字段中有两个字段，硬币和字段coin_charge，这里是屏幕快照弹出式镜头，我希望系统检查用户是否单击“是”，这是在尝试登顶和继续，让系统通过他的数据库数据库屏幕快照检查并启动这个论点(如果字段--硬币-小于字段--硬币_弹出式镜头-)应该弹出一个窗口，上面写着：“基金不足以在你的账户上继续前进，如果硬币字段大于COIN_CHARGE字段，让系统从硬币中减去coin_charge，那就是，如果(硬币= 100)和(coin_charge = 50 )硬币变成50

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<?php
    $message_query = mysql_query("select * from student ")or die(mysql_error());
?>

<?php
    while ($row = mysql_fetch_assoc($results)) {
        echo $row['coin'] = $row['coin'] - $row['coin_charge'];
    }
?>

Answer 1

$mysqli = new mysqli("localhost", "username", "password", "dbname");
$userid = $_POST["userid"];

$stmt = $mysqli->prepare('SELECT * FROM student WHERE id = ?');
$stmt->bind_param('i', $userid);
$stmt->execute();

$result = $stmt->get_result();
$row = $result->fetch_assoc();
$stmt->close();

$total = $row['coin'] - $row['coin_charge'];
if($total > 0){
    $stmt = $mysqli->prepare("update student set coin = ? where id = ?");
    $stmt->bind_param("ii", $total, $userid);
    $stmt->execute();
    $stmt->close();
    echo $total;
}else{
    echo 'Insufficient Fund to proceed Top Up your account';exit;
}