Woocommerce sale items via URL

_wc-sale-items.php

<?php 

// Display sale items only when searching by /?sale=show
add_action('pre_get_posts','sale_items');
function sale_items($query) {
  if (!is_admin() && ( $query->is_post_type_archive( 'product' ) || $query->is_tax( get_object_taxonomies( 'product' ))) && isset($_GET['sale']) && $_GET['sale'] == 'show' ) {
   $meta_query = array(
     array(
       'key' => '_sale_price',
       'value' => 0,
       'compare' => '>',
       'type' => 'numeric'
       )
     );
   $query->set( 'meta_query' , $meta_query );
 }
}