code: public function toSql()
      {
          if (empty($this->update) === true) {
              throw new BadUse('update clause has not been defined (' . print_r($this, true) . ')');
          }
          $sql = 'UPDATE ' . $this->update . ' ';
          $sql .= 'SET ' . $this->set . ' ';
  
          if ($this->where != null) {
              $sql .= 'WHERE ' . $this->where->toSql() . ' ';
          }
          if ($this->orderBy != null) {
              $sql .= 'ORDER BY ' . $this->orderBy . ' ';
          }
          if ($this->limit != null) {
              $sql .= 'LIMIT ' . $this->limit . ' ';
          }
  
          return $sql;
      }
docs: Extract the corresponding SQL code.
  
  @throws BadUse if update clause has not been defined
  @return string generated SQL code