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