统计2021年10月每个退货率不大于0.5的商品各项指标

select product_id,
round(if(count(product_id) = 0,0,sum(if_click) / count(product_id)),3) ctr,
round(if(sum(if_click) = 0,0,sum(if_cart) / sum(if_click)),3) cart_rate,
round(if(sum(if_cart)  = 0,0,sum(if_payment) / sum(if_cart)),3) payment_rate,
round(if(sum(if_payment) = 0,0,sum(if_refund) / sum(if_payment)),3) refund_rate
from tb_user_event
where date_format(event_time,'%Y-%m') = '2021-10' 
group by product_id
having refund_rate <= 0.5
order by product_id

为什么having可以直接用select中的字段呀，having执行顺序不是在select前吗

为什么对这个表重新select用where筛选refund_rate<=0.5这个会报错呀