def get_tx_lines(conn, scenario_id, subscenarios, col, col_value):
    """
    Get tx_lines for which the column value of "col" is equal to "col_value".
    E.g. "get the tx_lines of operational type tx_dcopf".
    :param conn: database connection
    :param subscenarios: Subscenarios class objects
    :param col: str
    :param col_value: str
    :return: List of tx_lines that meet the criteria
    """

    c = conn.cursor()
    tx_lines = c.execute(
        """SELECT transmission_line
        FROM inputs_transmission_portfolios
        LEFT OUTER JOIN
        inputs_transmission_operational_chars
        USING (transmission_line)
        WHERE transmission_portfolio_scenario_id = {}
        AND transmission_operational_chars_scenario_id = {}
        AND {} = '{}';""".format(
            subscenarios.TRANSMISSION_PORTFOLIO_SCENARIO_ID,
            subscenarios.TRANSMISSION_OPERATIONAL_CHARS_SCENARIO_ID,
            col, col_value
        )
    )
    tx_lines = [tx[0] for tx in tx_lines]  # convert to list
    return tx_lines