code: public function groupBy($groupBy)
      {
          if (is_array($groupBy) === true) {
              $this->groupBy = implode(', ', $groupBy);
          } else {
              $this->groupBy = $groupBy;
          }
  
          return $this;
      }
docs: Set group by clause.
  
  @param  array|string $groupBy group by clause
  @return Select       this