Use AJAX in a WordPress Plugin to Get Data From Custom Database?

You can use like this for send ajax…

function my_action_javascript()
{
?>
    <script type="text/javascript">
        jQuery(document).ready(function ($) {

            var data = {
                'action': 'my_action',
                'whatever': '1'
            };

            $.post(ajaxurl, data, function (response) {
                if ($('#menu_order').val() == '' || $('#menu_order').val() == '0')
                    $('#menu_order').val(response);
            });
        });
    </script> <?php
}

add_action('admin_footer', 'my_action_javascript'); 

function my_action_callback()
{
    $whatever = intval($_POST['whatever']);
    //$whatever += 1;
    echo $whatever;
    wp_die(); // this is required to terminate immediately and return a proper response
}
add_action('wp_ajax_my_action', 'my_action_callback');