zip code search

<?php
/*
Plugin Name: Zip Code Real-time Search
Description: Real-time zip code searc from database.
Version: 1.1

*/

add_action('init',function(){
    function live_search_function(){
        $content = ' <div id="livesearch_box">

                    <label id="zipcode_label">Zip Code:</label> <input type="text" id="zip_code" name="zip_code">
                    <label id="flatrate_label">Flat Rate:</label> <input type="text" id="flat_rate" name="flat_rate" readonly>
                    <input type="submit" value="Book Now" id="booknow-ls">
        </div>';
        return $content;
    }
    add_shortcode('live_search', 'live_search_function');
});


add_action('wp_head', 'myplugin_ajaxurl');
function myplugin_ajaxurl() {

   echo '<script type="text/javascript">
           var ajaxurl = "' . admin_url('admin-ajax.php') . '";
         </script>';
}

add_action('wp_footer', 'my_action_javascript');  // Write our JS below here
function my_action_javascript() { ?>
    <?php wp_enqueue_script('jquery'); ?>
    <!-- <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script> -->

    <script type="text/javascript" >
    jQuery( document ).ready(function() {
        jQuery( "#zip_code" ).keyup(search_key_press);
    });



    function search_key_press(){
            code = jQuery("#zip_code").val();
            var data = {
                'action': 'my_action_search_key_press',
                'code': code
            };
            jQuery.ajax({
                url: ajaxurl,
                method: "POST",
                data: data,
                dataType: "json",
                success: function(result){
                    if(result!="" || result!=null){
                        jQuery("#flat_rate").val(result);
                    }

                }
            });

    }

    </script> <?php
}

add_action( 'wp_ajax_my_action_search_key_press', 'action_search_key_press_callback' );

function action_search_key_press_callback() {
    global $wpdb;
    $code =  $_POST['code'];
    $table = "wp_zip";
    $sql = "SELECT flat_rate FROM wp_zip WHERE zip_code = '$code'";
    $rate = $wpdb->get_var($sql);
    echo json_encode($rate);
    wp_die();

    // wildcard searc
    // LIKE '%$code%'";
}