如何编写更新查询来减去某个值？

在 sql 查询中准备语句的情况下，这是执行减法运算的正确方法吗？

$sql = "UPDATE users set credits = (credits-$price) WHERE username = ?";

根据 $price 的值减去用户积分的代码

$price = $row0['price'];
    
    $sql = "UPDATE users set credits = (credits-$price) WHERE username = ?;";
        $stmt1 = mysqli_stmt_init($conn);
        if(!mysqli_stmt_prepare($stmt1, $sql)) {
                $db_err = array("error" => "Database");
                echo json_encode($db_err);
            } else {
                mysqli_stmt_bind_param($stmt1, "s", $_SESSION['username']);
                mysqli_stmt_execute($stmt1);

您需要使用占位符$price变量以正确使用准备好的语句。连接值是never除非您能够将该值与可能值列表进行比较，否则是安全的。

$sql = "UPDATE users set credits = (credits - ?) WHERE username = ?;";

mysqli_stmt_prepare($stmt1, $sql);
mysqli_stmt_bind_param($stmt1, "ss", $price, $_SESSION['username']);
mysqli_stmt_execute($stmt1);

请注意，最好使用对象语法很多原因 https://phpdelusions.net/mysqli/mysqli_connect#procedural。这样做的方法如下：

$stmt1 = $mysqli->prepare("UPDATE users set credits = (credits - ?) WHERE username = ?");
$stmt1->bind_param("ss", $price, $_SESSION['username']);
$stmt1->execute();