public function where($column = null, $relation = null, $value = null, $link = false)
    {
        if (is_null($column) && is_null($relation) && is_null($value)) {
            $this->where = ' where 1 = 1 ';
        } else {
            if (!$link) {
                $this->where = " where $column $relation '$value' ";
            } else {
                $this->where = " where $column $relation $value ";
            }
        }
        //
        return $this;
    }