public function forUpdate($table_name, $glue = ',')
	{
		$this->type = 'forUpdate';

		if ($this->forUpdate === null)
		{
			$glue            = strtoupper($glue);
			$this->forUpdate = new QueryElement('FOR UPDATE', 'OF ' . $table_name, "$glue ");
		}
		else
		{
			$this->forUpdate->append($table_name);
		}

		return $this;
	}