What is refund rate, and how do you measure it in Metabase?

Refunded value joined back to the month each order was placed.

-- Value-based refund rate by the month the ORDER was placed,
-- not the month the refund landed.
WITH refunds_per_order AS (
  SELECT order_id, SUM(amount) AS refunded_amount
  FROM refunds
  GROUP BY order_id
)
SELECT
  date_trunc('month', o.created_at)               AS order_month,
  COUNT(*)                                        AS orders,
  SUM(o.net_total)                                AS revenue,
  SUM(COALESCE(r.refunded_amount, 0))             AS refunded,
  ROUND(100.0 * SUM(COALESCE(r.refunded_amount, 0))
        / NULLIF(SUM(o.net_total), 0), 2)         AS refund_rate_pct
FROM orders o
LEFT JOIN refunds_per_order r ON r.order_id = o.id
WHERE o.financial_status IN ('paid', 'partially_refunded', 'refunded')
GROUP BY date_trunc('month', o.created_at)
ORDER BY order_month;

Rank products by share of value refunded over the last 180 days.

-- Products ranked by share of value refunded, with a volume
-- floor so one bad order doesn't top the list.
WITH refunds_per_line AS (
  -- Collapse the many partial-refund rows a line item can accrue into one
  -- total. Without this, the LEFT JOIN fans out and every extra refund row
  -- re-counts line_total, understating the rate.
  SELECT order_id, line_item_id, SUM(refunded_amount) AS refunded_amount
  FROM refund_line_items
  GROUP BY order_id, line_item_id
)
SELECT
  li.product_name,
  COUNT(DISTINCT li.order_id)                     AS orders,
  SUM(li.line_total)                              AS line_revenue,
  ROUND(100.0 * SUM(COALESCE(rl.refunded_amount, 0))
        / NULLIF(SUM(li.line_total), 0), 2)       AS refund_rate_pct
FROM order_line_items li
LEFT JOIN refunds_per_line rl
  ON rl.order_id = li.order_id
 AND rl.line_item_id = li.id
WHERE li.created_at >= CURRENT_DATE - INTERVAL '180 days'
GROUP BY li.product_name
HAVING COUNT(DISTINCT li.order_id) >= 50
ORDER BY refund_rate_pct DESC
LIMIT 20;