SQL#139

Seemed like your denominator included all orders, we want to limit to the first orders only.
Made a slight change and it starts to work:
With first_deliveries as (
SELECT first_value(order_id) over (partition by courier_id order by delivered_at) as first_delivery_order_id
FROM food_delivery
)
SELECT
extract(year from a.delivered_at) as year,
extract(month from a.delivered_at) as mon,
COUNT(DISTINCT case when a.rating = 1 then first_delivery_order_id else NULL end )*100.0/COUNT(DISTINCT A.order_id) as percent
FROM food_delivery a, first_deliveries b
WHERE a.order_id = b.first_delivery_order_id
GROUP BY year, mon;