code: public function set($col, $value = ClauseInterface::NO_VALUE)
      {
          if (is_array($col)) {
              return $this->setWithArrayData($col);
          }
  
          // update column name
          if (!isset($this->clause_set[$col])) {
              $this->clause_set[$col] = true;
          }
  
          // store data by row
          if (ClauseInterface::NO_VALUE !== $value) {
              $this->clause_data[$this->clause_rownum][$col] = $value;
          }
  
          return $this;
      }
docs: Set column values
  
  @param  string|array $col
  @param  mixed value scalar or Select query
  @return self
  @access public