php - 撤消用户创建，写入数据库

Question

这是一个很小的学校项目。我对PHP和SQL非常陌生。我有一个表格，用户可以在其中输入所需的用户名和密码。此后，我试图检查我的数据库，以查看是否存在具有相同用户名的帐户。这是代码：

$sqlValidate = "SELECT id FROM userAccounnts WHERE username='$username'";
$resultValidate = $conn->query($sqlValidate);

$sql = "INSERT INTO userAccounts VALUES($id, '$username', '$password')";

if ($resultValidate != 0) {

    $date = new DateTime(); //this returns the current date time
    $time = $date->format('d-m-Y-H-i-s');

    $sqlErrorLog = "insert into errorLog (errorType, errorTime) values ('Failed to create new user, username already exists', '$time')";
    $result = $conn->query($sqlErrorLog);

    echo '<script type="text/javascript">'
    , 'alert("Sorry,an account with that Username already exists. Please choose another one.");
    window.location.href = "user.php";'
    , '</script>'
    ;  
}

首先，我查询数据库。这样做的目的是检查用户名等于从表单传递的用户名时是否存在id。此后，我使用if语句检查是否id不等于0（以查看是否存在这样的ID）。如果是这样，我将错误日志写入数据库。

该代码不起作用。它将完全忽略if语句，并将新用户帐户写入数据库。在此先感谢您的帮助。只要代码有效，我都不介意代码是否安全。

PS，此else语句没有if语句。

Answer 1

最后，我决定采用这种方法：

<?php
$id=10001;
$sql = "SELECT id FROM userAccounts ORDER BY id DESC";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
$row = $result->fetch_assoc();
$id = $row["id"] + 1; 
}

$username = $_REQUEST["emailInput"];
$password = $_REQUEST["passwordInput"];

//the following query is used to check if a username with the same 
//name as the username entered by the user already exists
$sqlValidate = "SELECT id FROM userAccounts WHERE username='$username'";


$result = $conn->query($sqlValidate); //running the query

//if the query returns results, the error will be inserted into the DB using the following code:
if ($result->num_rows > 0) {

    $date = new DateTime(); //this returns the current date time
    $time = $date->format('d-m-Y-H-i-s');

    //the following code queries the DB    
    $sqlErrorLog = "insert into errorLog (errorType, errorTime) values ('Failed to create new user, username already exists', '$time')";
    $result = $conn->query($sqlErrorLog);

    echo '<script type="text/javascript">'
    , 'alert("Sorry,an account with that Username already exists. Please choose another one.");
    window.location.href = "user.php";'
    , '</script>'
    ;
    return;
}

因此，我查看是否存在具有相同用户名的id。如果查询返回结果（如果是具有此类用户名的行），则它将撤销该请求，将其写入数据库，并发出JavaScript警报以通知用户。它运行良好，并成功写入数据库。