code: def test_flat_aggregate(self):
      import pandas as pd
  
      mytop = udtaf(Top2())
      t = self.t_env.from_elements(
          [
              (1, "Hi", "Hello"),
              (3, "Hi", "hi"),
              (5, "Hi2", "hi"),
              (7, "Hi", "Hello"),
              (2, "Hi", "Hello"),
          ],
          ["a", "b", "c"],
      )
      result = (
          t.select(t.a, t.c)
          .group_by(t.c)
          .flat_aggregate(mytop.alias("a"))
          .select(t.a)
          .flat_aggregate(mytop.alias("b"))
          .select(t.b)
          .to_pandas()
      )
  
      assert_frame_equal(
          result, pd.DataFrame([[7], [5]], columns=["b"])
      )
  
imports: import unittest
  from pandas.testing import assert_frame_equal
  from pyflink.common import Row
  from pyflink.table import expressions, ListView
  from pyflink.table.types import DataTypes
  from pyflink.table.udf import udf, udtf, udaf, AggregateFunction, TableAggregateFunction, udtaf
  from pyflink.testing import source_sink_utils
  from pyflink.testing.test_case_utils import PyFlinkBatchTableTestCase, PyFlinkStreamTableTestCase
label_desc: Use image node_id 2 for calling the StreamRowBasedOperationITTests obj's underlying member method code with example usage: obj.test_flat_aggregate() without return types