blah.php

<?php

function _foo_handle_ajax_request_for_db_data() {
  $path = parse_url (site_url());
	if ( parse_url($_SERVER['REQUEST_URI'], PHP_URL_PATH) === $path['path'] . '/sth' ) {
		status_header(200);
		
	  // do sth

		exit ;
	}
}

add_action('init', '_foo_handle_ajax_request_for_db_data');