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我有下一页，用户可以在上面创建一个新帐户。我现在的问题是如何确保用户插入我的 MySQL 数据库中不存在的用户名。该页面在http://webs.hogent.be/kevinbaeyens/gebruiker.html上可用 ，该页面的代码是：

<!doctype html>

<html class="">

<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Nieuwe gebruiker | Sociale buurt</title>
<link href="boilerplate.css" rel="stylesheet" type="text/css">
<link href="onzebuurt.css" rel="stylesheet" type="text/css">

<script type="text/javascript" src="jquery-1.7.1.min.js"></script>
<script type="text/javascript" language="javascript">
    window.onload = function() {
      document.getElementById("Username").focus();
    };

    function formulierValideren() {     
            if (document.getElementById('Username').value == '' || document.getElementById('Username').value == null)
            {
                alert ('Gebruikersnaam is verplicht.');
                document.getElementById('Username').style.borderColor = "red";
                return false;
            }
            else if (document.getElementById('Wachtwoord').value == '' || document.getElementById('Wachtwoord').value == null)
            {
                alert ('Wachtwoord is verplicht.');
                document.getElementById('Wachtwoord').style.borderColor = "red";
                return false;
            }
            else if (document.getElementById('Wachtwoord2').value == '' || document.getElementById('Wachtwoord2').value == null)
            {
                alert ('Bevestig wachtwoord.');
                document.getElementById('Wachtwoord2').style.borderColor = "red";
                return false;
            }
            else if (document.getElementById('Wachtwoord2').value != document.getElementById('Wachtwoord').value)
            {
                alert ('Wachtwoorden komen niet overeen.');
                document.getElementById('Wachtwoord2').style.borderColor = "red";
                return false;
            }
            else
            {
                var url = "http://localhost:8080/OnzeBuurt2/resources/gebruikers";

                var reminder = {};
                reminder.naam = jQuery.trim($("#Username").val());
                reminder.wachtwoord = jQuery.trim($("#Wachtwoord").val());

                var request = new XMLHttpRequest();
                request.open("POST", url);
                request.onload = function() {
                    if (request.status === 201) {
                        reminder.id = request.getResponseHeader("Location").split("/").pop();


                    } else {
                        console.log("Error creating reminder: " + request.status + " " + request.responseText);
                    }
                };

                request.setRequestHeader("Content-Type", "application/json");
                request.send(JSON.stringify(reminder));     




                var msg = "Registratie succesvol. Klik op OK om u aan te melden op de site.";
                if(confirm(msg)){
                setTimeout(function() {window.location.href = "http://localhost:8080/OnzeBuurt2/"})
                }

            }
            //end if
    }//end function


</script>
</head>
<body class="body2">
<div class="gridContainer clearfix">
  <div class="header2">
        <center>    
        Nieuwe gebruiker
        </center>
  </div>
        <div id="formulier2">
            <form method="post" name="form" action="">
                <p class="labels"><center>Gebruikersnaam *</center></p><input id="Username" type="text" name="Username" placeholder="Gebruikersnaam" size="50">
                <p class="labels"><center>Wachtwoord *</center></p><input id="Wachtwoord" type="password" name="Wachtwoord" placeholder="Wachtwoord" size="50">
                <p class="labels"><center>Bevestig wachtwoord *</center></p><input id="Wachtwoord2" type="password" name="Bevestig wachtwoord" placeholder="Bevestig wachtwoord" size="50">
                <br />
                <a href="index.html" style="text-decoration:none"><center><img id="return" name="jsbutton" src="return.png" alt="Terug" /></center></a>
                <br />
                <center><input id="bevestig" type="image" src="Bevestig.png" width="200"  height="50" border="0" alt="SUBMIT!" onclick="formulierValideren()"></center>
                <br />
            </form>
        </div>
</div>
</body>
</html>

你需要知道的一些注意事项：

1）因为这是学校的作业，我不能使用PHP。

2）我的数据库表'Gebruiker'中的主键是'ID'，'username'的字段是必须是唯一的字段

提前致谢！

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