def dummy_df():
    """Return dummy dataframe.

    Example:

        >>> from pyspark.sql import SparkSession
        >>> spark = SparkSession.builder.getOrCreate()
        >>> df = spark.createDataFrame(
        ...     [
        ...         ("2021-08-31", "2021-05-31 00:00:00", "A", "A", "A"),
        ...         ("2021-05-31", "2021-01-31 00:00:00", "B", "B", "B"),
        ...     ],
        ...     [
        ...         "effective_end_date", "effective_start_date",
        ...         "entity_id", "country_id", "iuc",
        ...     ],
        ... )