public function distinct($on = null) {
        if ($on === null) {
            if (empty($this->query['distinct']) || $this->query['distinct'] !== 'DISTINCT') {
                $this->query['distinct']= 'DISTINCT';
            } else {
                $this->query['distinct']= '';
            }
        } else {
            $this->query['distinct']= $on == true ? 'DISTINCT' : '';
        }
        return $this;
    }