public function unsigned()
    {
        switch ($this->type) {
            case Schema::TYPE_PK:
                $this->type = Schema::TYPE_UPK;
                break;
            case Schema::TYPE_BIGPK:
                $this->type = Schema::TYPE_UBIGPK;
                break;
        }
        $this->isUnsigned = true;
        return $this;
    }