public function columns($cols = '*', $correlationName = null)
    {
        if ($correlationName === null && count($this->_parts[self::FROM])) {
            $correlationNameKeys = array_keys($this->_parts[self::FROM]);
            $correlationName = current($correlationNameKeys);
        }

        if (!array_key_exists($correlationName, $this->_parts[self::FROM])) {
            /**
             * @see Zend_Db_Select_Exception
             */
            // require_once 'Zend/Db/Select/Exception.php';
            throw new Zend_Db_Select_Exception("No table has been specified for the FROM clause");
        }

        $this->_tableCols($correlationName, $cols);

        return $this;
    }