$select = SELECT COUNT( DISTINCT wpPost.ID ) FROM `wp_posts` wpPost RIGHT JOIN `wp_woocommerce_order_itemmeta` wpOrderItem ON wpOrderItem.`meta_value`='6246' WHERE wpPost.`post_status`='wc-completed' GROUP BY wpPost.`ID` ORDER BY wpPost.`post_date` DESC
$wpdb->get_results("$select");
echo $wpdb->num_rows;