final public function groupBy($by)
    {
        if (is_string($by)) {
            $by = explode(',', $by);
        }

        foreach ($by as $val) {
            $val = trim($val);

            if ($val != '') {
                $this->arrayGroupBy[] = $this->identifiers($val);

                if ($this->arrayCaching === TRUE) {
                    $this->arrayCacheGroupBy[] = $this->identifiers($val);
                    $this->arrayCacheExists[] = 'groupby';
                }
            }
        }
        return $this;
    }