public function selectDistinct()
    {
        if ( $this->selectString == null )
        {
            $this->selectString = 'SELECT DISTINCT ';
        }
        elseif ( strpos ( $this->selectString, 'DISTINCT' ) === false )
        {
            throw new ezcQueryInvalidException( 
                'SELECT',
                'You can\'t use selectDistinct() after using select() in the same query.'
            );
        }

        // Call ezcQuerySelect::select() to do the parameter processing
        $args = func_get_args();
        return call_user_func_array(
            array( $this, 'select' ),
            $args
        );
    }