select *
from (
select product_id, round(sum(if_click)/count(*), 3) ctr, 
round(sum(if_cart)/sum(if_click), 3) cart_rate, 
round(sum(if_payment)/sum(if_cart), 3) payment_rate, 
round(sum(if_refund)/sum(if_payment), 3) refund_rate
from tb_user_event
where year(event_time) = 2021 and month(event_time) = 10
group by product_id) t1
where t1.refund_rate < 0.5