SQL的联接/子查询

我正在努力解决以下问题：

查询在所有发票上花费低于平均金额50%的所有客户。对于每个客户，显示他们的姓名和花费金额，精确到6位小数。按花费的金额从高到低排列结果

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假设您希望客户花费低于发票平均值，请尝试以下方法：

SELECT customer_name, CAST(SUM(total_price) AS DECIMAL(20,6) AS amount
FROM customer c
INNER JOIN invoice i 
ON c.id = i.customer_id 
WHERE amount < (SELECT AVG(total_price) FROM i)
ORDER BY amount DESC

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基本思想是在聚合后使用having子句：

SELECT c.id, c.customer_name, SUM(total_price) AS amount
FROM customer c LEFT JOIN
     invoice i 
     ON c.id = i.customer_id 
GROUP BY c.id, c.customer_name
HAVING COALESCE(SUM(total_price), 0) < (SELECT AVG(total_price) FROM i) ;

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注意：这包括没有发票的客户。

您可以使用窗口功能计算发票表的客户合计和合计平均值，然后将其连接到客户表，如下所示：

  select customer_name,round(max(customer_total),6) as amount_spent
    from customer c
    inner join (
                    select  customer_id,sum(total_price)over(partition by customer_id) Customer_Total, avg(total_price)over() Total_average from invoice) i
        on c.id=i.customer_id            
    where customer_total<=total_average?2
    group by customer_name
    order by customer_total desc

  

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您正在使用哪个数据库管理系统？