code: public function exists($db = null)
      {
          if ($this->emulateExecution) {
              return false;
          }
          $command = $this->createCommand($db);
          $params = $command->params;
          $command->setSql($command->db->getQueryBuilder()->selectExists($command->getSql()));
          $command->bindValues($params);
          return (bool) $command->queryScalar();
      }
docs: Returns a value indicating whether the query result contains any row of data.
  @param Connection $db the database connection used to generate the SQL statement.
  If this parameter is not given, the `db` application component will be used.
  @return bool whether the query result contains any row of data.