此查询将提供所需的结果。它在子查询中执行所有需要的聚合 JOIN 将其发送到主表以提供所需的输出：

SELECT s.customerID
     , Item
     , Quantity
     , Price
     , Tot_Qua
     , Quantity * Price AS Tot_pri
     , Avg_Pri
FROM stock s
JOIN (SELECT customerID
           , SUM(Quantity) AS Tot_Qua
           , SUM(Quantity * Price) / SUM(Quantity) AS Avg_Pri
      FROM stock
      GROUP BY customerID) s1
    ON s1.customerID = s.customerID

输出：

customerID  Item    Quantity    Price   Tot_Qua     Tot_pri     Avg_Pri
cus1        A       3           4       8           12          3.375
cus1        B       2           3       8           6           3.375
cus1        C       3           3       8           9           3.375
cus2        A       3           3.5     6           10.5        2.75
cus2        C       3           2       6           6           2.75
cus3        D       6           4       6           24          4

在dbfiddle上演示

select  customerID, item, quantity, price,
            (select sum(quantity) from stock s2 where stock.customerID = s2.customerID ) as tot_qua,
            price * quantity as tot_pri,
            (select sum(quantity * price) / sum(quantity) from stock s2 where stock.customerID = s2.customerID) as `sumtotpri/tot_qua`
from stock
order by stock.customerID, stock.item;

在mysql 8+中，您只需使用窗口函数：

select s.*,
       sum(quan) over (partition by cusid) as tot_quan,
       (quan * price) as tot_price,
       sum(quan * price) over (partition by cusid) / sum(quan) over (partition by cusid) as ratio
from stock s;

在早期版本中，您将使用子查询或类似的机制。可能 join 以及 group by 最简单的方法是：

select s.*, s2.tot_quan, (s.quan * s.price) as tot_price,
       tot_tot_price / tot_quan as ratio
from stock s join
     (select cusid, sum(quan) as tot_quan,
            sum(quan * price) as tot_tot_price
      from stock s2
      group by cusid
     ) s2
     using (cusid);