WordPress Ajax returns ‘0’

There are quite a few similar problems I could find, but nothing helped, so have to post my own version of this question.

I have 2 php files:

• playing.php (contains the web form and sends the ajax call)
• plug_search.php (has the form processing code)

Before I started trying to use ajax, the form processing worked perfectly, the query worked as expected and the proper result was returned, based on the search parameters. Now, I want to use ajax, so the results are returned to the same page and it always returns ‘0’. I tried (seems like) everything I could, ruled out the most probable reasons (incorrect function/call name) – no luck. Feels like something very simple is missing, I suspect the problem is in the declaration of the function, but can’t see what’s wrong (it seems like the call never reaches processing function in plug_search.php). I stripped all the query code and just trying to return a simple string – still same ‘0’.
If you could help, I’d appreciate that a lot!

playing.php

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js" type="text/javascript"></script>

<script type="text/javascript">
   $(document).ready(function(){
      function search(){
         var plug=$("#autocomplete-dynamic").val();
         var ajaxurl = "<?php echo admin_url('admin-ajax.php'); ?>";
         if(plug!==""){
            $("#result").html();
            $.ajax({
                type: "GET",
                url: ajaxurl,
                data: {
                    action: 'ajax_return_search_results',
                    plug: 'country'
                },
                success:function(data){
                   alert(data);
                }
            });
         }
      }

      $("#button").click(function(){
         search();
      });

      $('#search').keyup(function(e) {
         if(e.keyCode == 13) {
            search();
         }
      });
   });
 </script>

plug_search.php

<?php

add_action( 'wp_ajax_ajax_return_search_results', 'myajax_return_search_results' );
add_action( 'wp_ajax_nopriv_ajax_return_search_results', 'myajax_return_search_results' );

function myajax_return_search_results() {
   echo "Success";
   die();
}

?>

plug_search.php – Full version

function myajax_return_search_results() {

$value = $_GET['plug'];
$value2 = $_GET['country'];

$sql = "SELECT name, image_url, amazon_url, plug_type FROM adapters_list WHERE plug_type = '$value' AND country LIKE '%$value2%'" or die("Error in the consult.." . mysqli_error($link));

$result = $link->query($sql);

while($row = mysqli_fetch_array($result)) { ?>
<div id="output-product" style="border: 1px solid #333; font-family: Helvetica, sans-serif; font-size: 20px;"><?php echo $row["name"] . "<br />";?>
<?php echo "</div>";

die();

}