How to retrieve a list of WooCommerce orders which use a particular coupon?

Your code is not working because by default WooCommerce does't store used coupon code in wp_postmeta table. It stores in wp_woocommerce_order_items table, under order_item_type => coupon and order_item_name => YOUR_CODE.

You have to first get all the Order ID(s), then you have to loop it to get the desired total, or tax or discount.

Here is the code:

function wh_getOrderbyCouponCode($coupon_code, $start_date, $end_date) {    global $wpdb;    $return_array = [];    $total_discount = 0;    $query = "SELECT        p.ID AS order_id        FROM        {$wpdb->prefix}posts AS p        INNER JOIN {$wpdb->prefix}woocommerce_order_items AS woi ON p.ID = woi.order_id        WHERE        p.post_type = 'shop_order' AND        p.post_status IN ('" . implode("','", array_keys(wc_get_order_statuses())) . "') AND        woi.order_item_type = 'coupon' AND        woi.order_item_name = '" . $coupon_code . "' AND        DATE(p.post_date) BETWEEN '" . $start_date . "' AND '" . $end_date . "';";    $orders = $wpdb->get_results($query);    if (!empty($orders)) {        $dp = ( isset($filter['dp']) ? intval($filter['dp']) : 2 );        //looping throught all the order_id        foreach ($orders as $key => $order) {            $order_id = $order->order_id;            //getting order object            $objOrder = wc_get_order($order_id);            $return_array[$key]['order_id'] = $order_id;            $return_array[$key]['total'] = wc_format_decimal($objOrder->get_total(), $dp);            $return_array[$key]['total_discount'] = wc_format_decimal($objOrder->get_total_discount(), $dp);            $total_discount += $return_array[$key]['total_discount'];        }//        echo '<pre>';//        print_r($return_array);    }    $return_array['full_discount'] = $total_discount;    return $return_array;}

Code goes in function.php file of your active child theme (or theme). Or also in any plugin php files.

USAGE

$orders = wh_getOrderbyCouponCode('my_code', '2016-09-17', '2016-10-07');echo 'Total Discount : ' . $orders['full_discount'];//print_r($orders);

Please Note:

All dates are in YYYY-MM-DD format.
print_r(array_keys(wc_get_order_statuses())); will output something like this:

Array(    [0] => wc-pending    [1] => wc-processing    [4] => wc-on-hold    [5] => wc-completed    [6] => wc-cancelled    [7] => wc-refunded    [8] => wc-failed)

Code is tested and works.

Hope this helps!