php不显示警报

Question

我遇到以下代码问题。根据他的说法，如果在用户名的地方，我们将输入数据库中缺少的数据，密码应显示自己的信息警报。不幸的是，如果我输入错误的数据模态消失并且没有显示警报。 我将不胜感激。

<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"]);  
 }  
 ?>