查询MySQL以检查用户是否已存在于两个表中

Question

我正在学习有关如何为网站创建用户注册系统的教程。我已经做到了这一点step，但是我需要检查两个表中的多个值以查看它们是否存在。本教程不执行此操作。

这是我的注册表：

<form action="/members/register/" name="registerForm">
    <div>
        <h2>Register</h2>
        <h6>welcome</h6>
        <div class="register">
            <input type="text" name="firstname" placeholder="First Name" required>
            <input type="text" name="lastname" placeholder="Last Name" required>
            <input type="text" name="email" placeholder="Email" required>
            <label for="dob">Date of Birth:
                <input type="date" name="dob" id="dob" placeholder="Date of Birth" required>
            </label>
            <input type="text" name="username" placeholder="Username" required>
            <input type="password" name="password" placeholder="Password" required>
            <input type="password" name="passwordconfirm" placeholder="Confirm Password" required>
            <p>By creating an account you agree to our <a href="/legal/terms-of-use/">Terms &amp; Privacy</a>.</p>
            <button type="submit" name="registerNow">Register</button>
        </div>
    </div>
</form>

我需要检查的mySQL数据库表是：

users
  -id
  -username
  -password

registered
  -id
  -nameFirst
  -nameLast
  -email
  -dob

我需要创建一个查询来检查是否存在以下任何一项：1）姓和名在一起，2）用户名，或3）电子邮件。

此外，一旦我了解了如何执行这样的查询，我对查询中的?仍然会有些困惑。另外，此代码示例仅检查用户名是否存在，并输出“用户名已存在”。请选择另一个。我需要根据表中已存在的字段输出不同的内容。这是教程中的代码：

if ($stmt = $con->prepare('SELECT id, password FROM accounts WHERE username = ?')) {
// Bind parameters (s = string, i = int, b = blob, etc), hash the password using the PHP password_hash function.
$stmt->bind_param('s', $_POST['username']);
$stmt->execute();
$stmt->store_result();
// Store the result so we can check if the account exists in the database.
if ($stmt->num_rows > 0) {
    // Username already exists
    echo 'Username already exists. Please choose another.';
} else {
    // Insert new account
}
$stmt->close();
} else {
// Something is wrong with the sql statement, check to make sure accounts table exists with all 3 fields.
echo 'Could not prepare statement!';
}
$con->close();

会是这样吗？

SELECT id, password 
FROM users, registered 
WHERE users.username = ? OR (registered.nameFirst = ? AND registered.nameLast = ?) OR registered.email = ?

同样，我正在学习如何使用本教程来执行此操作，因此，我不想就代码的操作方式对其进行任何更改。我了解有更好的方法可以做到这一点。我以此为出发点来学习和进步。

Answer 1

由于您要做的就是检查注册值是否已经存在，所以使用EXISTS子查询而不是将两个表一起JOIN可能是最简单的。像这样的东西：

SELECT
    EXISTS (SELECT * FROM users WHERE username = ?) AS found_username,
    EXISTS (SELECT * FROM registered WHERE nameFirst = ? AND nameLast = ?) AS found_name,
    EXISTS (SELECT * FROM registered WHERE email = ?) AS found_email

在此查询中，?代表用户名，名字，姓氏和电子邮件值的占位符。使用带占位符的预准备语句的目的是防止SQL注入，有关更多信息，请参见this Q&A。使用它们还具有消除在输入中转义特殊字符的需求（例如，如果您想使用单引号引起来的O'Hara插入nameLast字段）的好处。

因此，对于您的代码，您将执行以下操作：

if ($stmt = $con->prepare('SELECT
        EXISTS (SELECT * FROM users WHERE username = ?) AS found_username,
        EXISTS (SELECT * FROM registered WHERE nameFirst = ? AND nameLast = ?) AS found_name,
        EXISTS (SELECT * FROM registered WHERE email = ?) AS found_email')) {
    // Bind parameters (s = string, i = int, b = blob, etc)
    $stmt->bind_param('ssss', $_POST['username'], $_POST['firstname'], $_POST['lastname'], $_POST['email']);
    $stmt->execute();
    $stmt->bind_result($found_username, $found_name, $found_email);
    $stmt->fetch();
    // Store the result so we can check if the account exists in the database.
    if ($found_username) {
        // Username already exists
        echo 'Username already exists. Please choose another.';
    }
    elseif ($found_name) {
        // Name already exists
        echo 'Name already exists. Please choose another.';
    }
    elseif ($found_email) {
        // Email already exists
        echo 'Email already exists. Please choose another.';
    }
    else {
        // Insert new account
    }
    $stmt->close();
} 
else {
    // Something is wrong with the sql statement, check to make sure accounts table exists with all 3 fields.
    echo 'Could not prepare statement!';
}
$con->close();

Answer 2

我认为您希望您的SQL语句使用这样的JOIN，以避免重复的行匹配：

    SELECT 
      users.id, password 
    FROM 
      users JOIN registered 
    USING(id) 
    WHERE 
      username = ? 
      OR (nameFirst = ? AND nameLast = ?) 
      OR email = ?

然后，您需要将其他参数添加到绑定中：

$stmt->bind_param(
   'ssss', 
   $_POST["username"], 
   $_POST["firstname"], 
   $_POST["lastname"], 
   $_POST["email]
);
$stmt->execute();
// etc.

Answer 3

？将会改变。请记住，如果出现双重和三重逗号，我会避免出现问题