code: public function scalar($db = null)
      {
          if ($this->emulateExecution) {
              return null;
          }
  
          return $this->createCommand($db)->queryScalar();
      }
docs: Returns the query result as a scalar value.
  The value returned will be the first column in the first row of the query results.
  @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 string|null|false the value of the first column in the first row of the query result.
  False is returned if the query result is empty.