How to show message, in jquery by fetching the value and match and then insert value and show jquery message

How to show message, in jquery by fetching the value and match and then insert value and show jquery message

THIS IS first_page

<html>
<head>
function checkAvailability() 
{
jQuery.ajax({
url: "insert.php",
data:'sub_name ='+$("#sub_name").val(),
//data:'sub_mark='+$("#sub_mark").val(),
type: "POST",
success:function(data)
{
$("#user-availability-status").html(data);
},
error:function ()
{
}
});
}

</script>
</head>
<body>
      <form class="" name="" action="insert.php" method="post" >
<fieldset>
<!-- Subject Name input-->
<div id="frmCheckUsername"></div>
<div class="form-group">
<span id="user-availability-status"></span>
<label class="col-md-3 control-label" for="name">Subject Name</label>
<div class="col-md-9">
<select class="form-control" name="sub_name" id="sub_name">
<option value="-1" selected>Choose Subject</option>
<option value="English">English</option>
<option value="Bengali">Bengali</option>
<option value="Mathmatics">Mathmatics</option>
<option value="General Knowledge">General Knowledge</option>
</select>
</div><span id="user-availability-status"></span>
</div>
<!-- Subject Mark input-->
<div class="form-group">
<label class="col-md-3 control-label" for="number">Mark</label>
<div class="col-md-9">
<input id="sub_mark" name="sub_mark" type="number" placeholder="Please Input Mark" class="form-control" >
</div>
</div>
<!-- Form actions -->
<div class="form-group">
<div class="col-md-12 widget-right">
<button type="submit" id="sub_add" class="btn btn-default btn-md pull-right" name="add_subject" style="color: #333333; background-color: #0099CC; width: 100px;" onClick="checkAvailability()">Add</button>
</div>
</div>
</fieldset>
</form>
</body>

THIS IS INSERT.PHP PAGE
<?php
require_once("subcontroller.php");
$db_handle = new DBController();

if(isset($_POST['add_subject']) || (!empty($_POST['sub_name'])))
{
$result = mysql_query("SELECT count(*) FROM test WHERE subject ='" . $_POST["sub_name"] . "'");
$row = mysql_fetch_row($result);
$user_count = $row[0];
if($user_count>0) 
{
echo "<span class='status-not-available'> Subject Already Exist.</span>";
}
else
{
$result = mysql_query("INSERT INTO test (subject, mark) VALUES ('".$_POST["sub_name"]."','".$_POST["sub_mark"]."')");
if($result)
{
echo "<span class='status-available'> New Subject Inserted.</span>";
}
 
}
}
?>

and the last page subcontroller.php
<?php
class DBController {
private $host = "localhost";
private $user = "root";
private $password = "";
private $database = "examination";
function __construct() {
$conn = $this->connectDB();
if(!empty($conn)) {
$this->selectDB($conn);
}
}
function connectDB() {
$conn = mysql_connect($this->host,$this->user,$this->password);
return $conn;
}
function selectDB($conn) {
mysql_select_db($this->database,$conn);
}
function runQuery($query) {
$result = mysql_query($query);
while($row=mysql_fetch_assoc($result)) {
$resultset[] = $row;
}
if(!empty($resultset))
return $resultset;
}
function numRows($query) {
$result  = mysql_query($query);
$rowcount = mysql_num_rows($result);
return $rowcount;
}
}
?>