正在学习无需刷新表单验证 碰到了一些mysql数据库问题 求解答

正在学习无需刷新表单验证 碰到了一些问题 求解答
应该如何编写SQL语句啊

表单：
<form action="" method="post" name="form1" id="form1">
    <fieldset>
    <legend>账号基本数据</legend>
    <label>账号：</label>
    <input name="ID" type="text" onblur="sendRequest(this.value);" size="40" />
    <span id="chkr"></span><br />
    <label>密码：</label>
    <input name="rPWD" type="password" size="40" />
    <br />
    <label>&nbsp;</label>
    <span class="style1">a9527,是个重复账号</span>
    </fieldset>
</form>
<?
$mysql_server_name = "localhost";
$mysql_username    = "root";
$mysql_password    = "123";
$mysql_database    = "register";
mysql_connect( $mysql_server_name, $mysql_username, $mysql_password )
    or die("database communication failed");
mysql_select_db($mysql_database);
//  end of 开启资料库   
$message = "";
$ID = $_GET['ID'];
$query = "SELECT username FROM users WHERE username = '$ID'";
$result = mysql_query($query);
if( mysql_num_rows($result) != 1 )
{
    //  The user ID specified was not found
    $message = "False";
}
else
{
    //  The user ID found
    $message = "True";
}
?>
<?php echo $message; ?>
真心求教