最简单的代码方式

select
    round(avg(total_amount),1) as avg_amount,
    round(avg(sum_cost-total_amount),1) as avg_cost
from tb_order_overall 
    join(select order_id,sum(price*cnt) as sum_cost 
        from tb_order_detail 
        group by order_id
        )  as t  using(order_id)
where event_time like '2021-10%'#2021年10月
 and (uid,event_time) in (select uid, min(event_time) as new from tb_order_overall group by uid) #新用户的首单

直接在join中把订单总金额汇总。按照订单来做表关联。

求平均，直接avg就可以，不用sum之后在除。