mySQL查询问题 - 作业

Question

我需要一些帮助来解决我的作业中的一个问题，我现在已经尝试了大约一个小时而且无法让它运行。

列出购买的商品数量超过每位顾客的平均商品数量

表格如下：

Customer(Cnum, CustomerName, Address)  
Item(Inum, ItemName, Manufacturer, Year)  
Bought(Cnum, Inum, Date, Quantity)   
Prefer(Inum, Cnum)   

我能想到的最好的是，它需要是每个客户的总数量与数量的总体平均值相比较。我尝试过各种形式的查询：

SELECT Cnum
FROM Bought
WHERE
(
    SELECT Cnum, SUM(Quantity)
    FROM Bought
    GROUP BY Cnum;
) >
(
    SELECT AVG(Quantity)
    FROM Bought
);

但是它返回一个错误 - （phpMyAdmin没有告诉我问题是什么，只是没有执行并且没有连接页面，这意味着我的查询中出错）

我还尝试使用以下命令返回更高的SUM：

SELECT SUM(Quantity)
FROM Bought
WHERE SUM(Quantity) > AVG(Quantity);

同样的问题。

任何帮助都会受到赞赏，甚至可以解释第二个失败的原因。

Answer 1

您可能需要查看SQL的HAVING子句。

注意：由于这是家庭作业，我故意不给你完整的答案。

Answer 2

为什么不试试这个。

SELECT `Cnum` , Sum( Quantity )
FROM `bought`
GROUP BY `Cnum`
HAVING Sum( Quantity ) > ( SELECT AVG( Quantity ) FROM Bought )

Answer 3

尝试这可能有帮助

SELECT Cnum, SUM(Quantity) 
FROM Bought
GROUP BY  Cnum
HAVING SUM(OrderQuantity) > avg(Quantity)

Answer 4

尝试以下：

SELECT Cnum
FROM Bought
having SUM(Quantity) > (SELECT avg(Quantity) FROM Bought)
order by SUM(Quantity) desc

Answer 5

试试这个

SELECT Cnum, Inum, SUM(Quantity) sum, AVG(QUANTITY) average
from bought group by cnum,inum having sum > average