php - HTML PHP - 如何检查用户名是否已经存在

Question

该脚本已经工作正常，但我想插入一个仅在用户名尚未使用时才允许的命令。

if (isset($_POST['submit'])) {
    $firstname = htmlentities($_POST['firstname'], ENT_QUOTES);
    $lastname = htmlentities($_POST['lastname'], ENT_QUOTES);
    $position = htmlentities($_POST['position'], ENT_QUOTES);
    $username = htmlentities($_POST['username'], ENT_QUOTES);
    $password = htmlentities($_POST['password_two'], ENT_QUOTES);
    $uniqid = uniqid('', true);
    if ( $firstname == '' || $lastname == '' ||  $position == '' || $username == '' || $password == '') {
        $error = 'ERROR: Please fill in all required fields!';
        renderForm($error, $firstname, $lastname, $position, $username, $password);

    } else {
        if ($stmt = $connection->prepare("INSERT INTO employee (uniqid, firstname, lastname, position, username, password) VALUES (?, ?, ?, ?, ?, ?)")) {
            $stmt->bind_param("ssssss", $uniqid, $firstname, $lastname, $position, $username, $password);
            $stmt->execute();
            $stmt->close();
        } else {
            echo "ERROR: Could not prepare SQL statement.";
        }
        header("Location: regemployee.php");
    }
} else {
    renderForm();
}

Answer 1

使用户名在数据库中唯一，然后当您尝试再次将相同的用户名插入数据库时​​，插入将出错。

或者，您可以执行 SELECT * FROMEmployee WHERE username = ? 并检查结果是否 > 0。

然后你就会知道它已经存在。