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 $wpdb->pmpro_discount_codes as dc INNER JOIN $wpdb->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');

Note that PMPro shows the discount code on the orders table by default now and this gist is not needed.

If you want to see an example of how to add columns to the orders table see here:
https://gist.github.com/ideadude/6a5b60ee19cc8a338eda5168fa1aedca

This old gist has a few problems: (1) The ob_start/ob_get_clean stuff is not needed and messes this gist up. (2) These are also action hooks, not filters.