Question

我的invoices表格有orderId列，该列是引用orderId primary key column of orders table的外键，订单表格有customerId列，引用customerId主键列客户表。客户可以拥有多个订单，但订单只有一张发票。

我想计算每个客户的发票数量。以下是我试过的查询：

SELECT customers.name, COUNT(*) as number_of_invoices 
FROM invoices 
JOIN orders 
ON invoices.orderId = orders.orderId 
JOIN customers 
ON orders.customerId = customers.customerId;

但它只返回给我一个客户，计数是总数而不是该客户的发票数。

Answer 1

以下是您的问题的解决方案：

SELECT customers.name, COUNT(*) as number_of_invoices 
FROM invoices 
INNER JOIN orders 
ON invoices.orderId = orders.orderId 
INNER JOIN customers 
ON orders.customerId = customers.customerId
GROUP BY Customers.name;

只需在查询中添加GROUP BY，即可为每位客户分组发票组。

Answer 2

如果您只需要了解每位客户的发票数量。可以在没有JOIN的情况下完成。由于订单和发票具有一对一的映射，您可以使用以下sql查询

select customerId ,count(orderId) as invoice_count from orders group by customerId

使用mysql连接来计算客户的发票数量

2 个答案: