def get_data_types_query() -> str:
    """
    Define custom data types inside a database.
    """
    # Define data types.
    query = """
    CREATE TYPE AssetClass AS ENUM ('futures', 'etfs', 'forex', 'stocks', 'sp_500');
    CREATE TYPE Frequency AS ENUM ('minute', 'daily', 'tick');
    CREATE TYPE ContractType AS ENUM ('continuous', 'expiry');
    CREATE SEQUENCE serial START 1;
    """
    return query