public function concat()
    {
        $args = func_get_args();
        $cols = ezcQuerySelect::arrayFlatten( $args );

        if ( count( $cols ) < 1 )
        {
            throw new ezcQueryVariableParameterException( 'concat', count( $args ), 1 );
        }

        $cols = $this->getIdentifiers( $cols );
        return join( ' + ', $cols );
    }