code: public function exists($db = null)
      {
          if ($this->emulateExecution) {
              return false;
          }
          return $this->one($db) !== null;
      }
docs: Returns a value indicating whether the query result contains any row of data.
  @param Connection $db the database connection used to execute the query.
  If this parameter is not given, the `db` application component will be used.
  @return bool whether the query result contains any row of data.