Woocommerce Filter by On Sale

woocommerce-filter-on-sale.php

<?php 

/*
 * Plugin Name:       On Sale Filter for WooCommerce Admin 
 * Description:       Filter the products list in WooCommerce admin to show items on sale.
 * Version:           1.0
 * Requires at least: 5.2
 * Requires PHP:      7.2
 */

// Credit: https://wpsimplehacks.com/how-to-filter-woocommerce-admin-products-by-on-sale/

// Filter Woocommerce admin product by on sale

function wpsh_onsale_filter($output) {
    global $wp_query;
    $selected = filter_input(INPUT_GET, 'product_sale', FILTER_VALIDATE_INT);
    if ($selected == false) {
        $selected = 0;
    }
    
    $output .= '
        <select id="dropdown_product_sale" name="product_sale">
            <option value="">Filter by sale</option>
            <option value="1" ' . (($selected === 1) ? 'selected="selected"' : '') . '>On Sale</option>
            <option value="2" ' . (($selected === 2) ? 'selected="selected"' : '') . '>No Sale</option>
        </select>
    ';
 
    return $output;
}
add_action('woocommerce_product_filters', 'wpsh_onsale_filter');
 
// Woocommerce Filter by on sale where statement

function wpsh_onsale_filter_where_statement($where) {
    global $wp_query, $wpdb;
 
    // Get selected value
    $selected = filter_input(INPUT_GET, 'product_sale', FILTER_VALIDATE_INT);
    
    // Only trigger if required
    if (!is_admin() || get_query_var('post_type') != "product" || !$selected) {
        return $where;
    }
	
	
	$todaytimestamp = strtotime(date('Y-m-d', time()) . ' 23:59:59');
 
    $querystr = '
            SELECT p.ID, p.post_parent
            FROM ' . $wpdb->posts . ' p
            WHERE p.ID IN (
                SELECT post_id FROM ' . $wpdb->postmeta . ' pm WHERE pm.meta_key = "_sale_price" AND pm.meta_value > \'\'
            ) AND p.ID IN (
                SELECT post_id FROM ' . $wpdb->postmeta . ' pm WHERE pm.meta_key = "_sale_price_dates_from" AND pm.meta_value <= '.$todaytimestamp.' 
            ) AND p.ID IN (
                SELECT post_id FROM ' . $wpdb->postmeta . ' pm WHERE pm.meta_key = "_sale_price_dates_to" AND pm.meta_value >= '.$todaytimestamp.' 
            )
        ';
        
        $pageposts = $wpdb->get_results($querystr, OBJECT);
        
        $productsIDs = array_map(function($n){
            return $n->post_parent > 0 ? $n->post_parent : $n->ID;
        }, $pageposts);

    if ($selected == 1) {
        $where .= ' AND ' . $wpdb->posts . '.ID IN (' . implode(",", $productsIDs) . ') ';
    } 
    elseif ($selected == 2) {
        $where .= ' AND ' . $wpdb->posts . '.ID NOT IN (' . implode(",", $productsIDs) . ') ';
    }
        
    return $where;
}
add_filter('posts_where' , 'wpsh_onsale_filter_where_statement');