select
sum(t1.sales_num*t2.goods_price) sales_total
,round(sum(t1.sales_num*t2.goods_price)/count(distinct t1.user_id),3) per_trans 
from sales_tb t1
join goods_tb t2
on t1.goods_id = t2.goods_id
where t1.sales_date between '2021-12-21' and '2021-12-31'