在 sql 中查找整个列的平均值

Question

我正在尝试创建一个包含总体平均评分和卖家平均评分的表格。我得到了卖家的评价，但没有得到整体评价。附件也是表格的样本； ()

到目前为止，这是我的代码

select rating_for_user_id, u.user_firstname, u.user_lastname,  count(r.rating_for_user_id) as number_of_ratings_seller_has, 
CAST( avg(r.rating_value) AS DECIMAL(4,2)) as avg_seller_rating
from vb_user_ratings r
join vb_users u on u.user_id= r.rating_for_user_id
where rating_astype= 'Seller'group by u.user_firstname, u.user_lastname, rating_for_user_id