public function is_existing_sku($product_id, $sku)
{
global $wpdb;
return $wpdb->get_var($wpdb->prepare("\n\t\t\tSELECT {$wpdb->posts}.ID\n\t\t\tFROM {$wpdb->posts}\n\t\t\tLEFT JOIN {$wpdb->postmeta} ON ( {$wpdb->posts}.ID = {$wpdb->postmeta}.post_id )\n\t\t\tWHERE {$wpdb->posts}.post_type IN ( 'product', 'product_variation' )\n\t\t\tAND {$wpdb->posts}.post_status = 'publish'\n\t\t\tAND {$wpdb->postmeta}.meta_key = '_sku' AND {$wpdb->postmeta}.meta_value = '%s'\n\t\t\tAND {$wpdb->postmeta}.post_id <> %d LIMIT 1\n\t\t ", wp_slash($sku), $product_id));
}