public function command($command= 'SELECT') {
        $command= strtoupper(trim($command));
        if (preg_match('/(SELECT|UPDATE|DELETE)/', $command)) {
            $this->query['command']= $command;
            if (in_array($command, array('DELETE','UPDATE'))) $this->_alias= $this->xpdo->getTableName($this->_class);
        }
        return $this;
    }