Question

表架构：

customers (customerID: integer, fName: string, lName: string)
orders (orderID: integer, itemID: integer, aID: integer, customerID: integer, date: date)
items (itemID: integer, description: string, price: integer)

SQL：

SELECT customers.customerID, COUNT(orders.customerID)
FROM customers INNER JOIN orders
    ON customers.customerID = orders.customerID
GROUP BY customers.customerID

我如何显示每位客户的订单数量结果？当我使用GROUP BY时，它会移除COUNT(orders.customerID)为NULL的行。有没有办法显示0？

Answer 1

不是过滤掉它的GROUP BY。这是INNER JOIN。

您正在寻找LEFT JOIN：

select customers.customerID,
    COUNT(orders.customerID)
from customers
left join orders on customers.customerID = orders.customerID
group by customers.customerID

使用GROUP BY（sql）时使用COUNT显示0

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