code: def test_select_returns_only_selected_columns(self):
      api_handler = Mock(RocketChatHandler)
      api_handler.call_rocket_chat_api.return_value = pd.DataFrame(
          [
              [
                  "message_id_1",  # id
                  "GENERAL",  # room_id
                  "bot_id_1",  # bot_id
                  "YEWWWWW",  # text
                  "shoresey",  # username
                  "Shore Keeso",  # name
                  "2023-05-05T00:31:46.825Z",  # sent_at
              ]
          ]
      )
      messages_table = RocketChatMessagesTable(api_handler)
  
      room_id_identifier = Identifier(path_str="room_id")
      text_identifier = Identifier(path_str="text")
      select_basic = ast.Select(
          targets=[room_id_identifier, text_identifier],
          from_table="channel_messages",
          where='room_id = "GENERAL"',
      )
  
      all_messages = messages_table.select(select_basic)
      first_message = all_messages.iloc[0]
  
      self.assertEqual(all_messages.shape[1], 2)
      self.assertEqual(first_message["room_id"], "GENERAL")
      self.assertEqual(first_message["text"], "YEWWWWW")
  
file_path: mindsdb/tests/handler_tests/test_rocket_chat_handler.py
imports: from mindsdb.integrations.handlers.rocket_chat_handler.rocket_chat_tables import RocketChatMessagesTable
  from mindsdb.integrations.handlers.rocket_chat_handler.rocket_chat_handler import RocketChatHandler
  from mindsdb_sql.parser import ast
  from mindsdb_sql.parser.ast.select.star import Star
  from mindsdb_sql.parser.ast.select.identifier import Identifier
  from unittest.mock import Mock
  import pandas
  import unittest
label_desc: Use image node_id 3 for calling the RocketChatMessagesTableTest obj's underlying member method code with example usage: obj.test_select_returns_only_selected_columns() without return types