PHP不显示警报

Question

I have a problem with the following code. According to him if in the place on username and we will enter data which are missing in the database on the password should show it oneself alert with information. Unfortunately if I will enter wrong data modal is disappearing and alert isn't being shown. I will be grateful for help.

<script> $(document).ready(function(){  
  $('#login_button').click(function(){  
       var username = $('#username').val();  
       var password = $('#password').val();  
       if(username != '' && password != '')  
       {  
            $.ajax({  
                 url:"action.php",  
                 method:"POST",  
                 data: {username:username, password:password},  
                 success:function(data)  
                 {  
                      //alert(data);  
                      if(data == 'No')  
                      {  
                           alert("Wrong Data");  
                      }  
                      else  
                      {  
                           $('#loginModal').hide();  
                           location.reload();  
                      }  
                 }  
            });  
       }  
       else  
       {  
            alert("Both Fields are required");  
       }  
  });  
  $('#logout').click(function(){  
       var action = "logout";  
       $.ajax({  
            url:"action.php",  
            method:"POST",  
            data:{action:action},  
            success:function()  
            {  
                 location.reload();  
            }  
       });  
  });   });  

AND Action.php

 <?php  
 session_start();  
 $connect = mysqli_connect("localhost", "root", "", "testing");  
 if(isset($_POST["username"]))  
 {  
      $query = "  
      SELECT * FROM admin_login  
      WHERE admin_name = '".$_POST["username"]."'  
      AND admin_password = '".$_POST["password"]."'  
      ";  
      $result = mysqli_query($connect, $query);  
      if(mysqli_num_rows($result) > 0)  
      {  
           $_SESSION['username'] = $_POST['username'];  
           echo 'Yes';  
      }  
      else  
      {  
           echo 'No';  
      }  
 }  
 if(isset($_POST["action"]))  
 {  
      unset($_SESSION["username"]);  
 }  
 ?>