public function groupBy($fields)
    {
        if (!is_array($fields)) {
            $fields = $this->splitParts($fields);
        }

        foreach ($fields as $k => $field) {
            $fields[$k] = $this->db->quoteColumn($field);
        }

        $this->query['group'] = implode(',', $fields);

        return $this;
    }