code: protected function buildInsertSet()/*# : array */
      {
          $result = [];
  
          if (empty($this->clause_set)) {
              return $result;
          }
  
          // build cols first
          $cols = [];
          foreach (array_keys($this->clause_set) as $col) {
              $cols[] = $this->quote($col);
          }
          $result[] = '(' . join(', ', $cols) . ')';
  
          return $result;
      }
docs: Build ( ... )
  
  @return array
  @access protected