Is there an open issue for this PR?

Hi Dmytro, There's not currently an open issue. We just saw an issue recently after updating our fork. Should I create an issue first?

Please add a sample code that does not work in the current Floor version and a relevant explanation of the issue.
It'll help reviewers understand what problem this PR solves.

Floor version 1.4.2
Dart 2.19.6
Flutter 3.7.12

Issue:
When performing a queryList with a raw SELECT UNION directly with QueryAdapter, the query fails and throws a StateError. Since sqlparser returns the rootNode as a CompoundSelectStatement, the type check for SelectStatement fails.

Fix:
To fix the issue and allow other types of SELECT statements, it should instead check for the BaseSelectStatement type which is extended by both SelectStatement and CompoundSelectStatement.

Sample Code
The code below throws a StateError when added to the floor example project.

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();

  final database = await $FloorFlutterDatabase
      .databaseBuilder('flutter_database.db')
      .build();
  final dao = database.taskDao;

  var qa = QueryAdapter(database.database);
  var list = await qa.queryList(
      'SELECT message FROM `Task` UNION SELECT message FROM `Task`',
      mapper: (e) => Task.optional(message: e['message'] as String?));
  
  runApp(FloorApp(dao));
}

@mthongvanh
thank you for details
yes, I can see the problem, but we need to verify if we're breaking anything with this change.
Meanwhile, Floor now supports Dart types, so the following approach can be used to retrieve the list of messages in the DAO.

@Query('SELECT message FROM Task UNION SELECT message FROM Task')
Future<List<String>> queryMessages();