Add discount code column to order list

discounts.php

function my_pmpro_add_cols_header($order_ids) {

    ob_start(); ?>
    <th>Discount Code</th><?php

    return ob_get_clean();
}
add_filter('pmpro_orders_extra_cols_header', 'my_pmpro_add_cols_header');

function my_pmpro_add_cols_body($order) {

    global $wpdb;

    $sql = "SELECT code from bbwp_pmpro_discount_codes as dc INNER JOIN bbwp_pmpro_discount_codes_uses as dcu ON (dcu.code_id = dc.id) WHERE dcu.order_id=%s";

    $discount_code = $wpdb->get_var($wpdb->prepare($sql, $order->id));
    ob_start(); ?>

    <td><?php echo $discount_code; ?></td>

    <?php
    return ob_get_clean();
}

add_filter('pmpro_orders_extra_cols_body', 'my_pmpro_add_cols_body');