mysql:每个子查询结果返回列

Question

我有 3 个表:customers、orders 和 order-items。该问题要求编写一个 SELECT 语句，该语句将返回一个带有

的表

• 客户的电子邮件地址
• 该客户的订单数量
• 以及每个订单的总费用

前两点相当简单:

SELECT email_address,
       (SELECT SUM(order_id) 
        FROM orders 
        WHERE orders.customer_id = cust.customer_id) as "order_count"
FROM customers as cust

我知道我可以获取每个订单的 order_total，如下所示:

SELECT order_id, 
    (SELECT SUM((item_price - discount_amount) * quantity)
     FROM order_items 
     WHERE order_items.order_id = ord.order_id) as "order_total"
FROM orders AS ord

但是，如果我尝试将它们结合起来并执行以下操作:

SELECT email_address,
       (SELECT SUM(order_id)
        FROM orders AS ord
        WHERE ord.customer_id = cust.customer_id) as "order_count",
       (SELECT(SELECT SUM((item_price - discount_amount) * quantity)
         FROM order_items 
         WHERE order_items.order_id = ord.order_id)
         FROM orders AS ord
         WHERE ord.customer_id = cust.customer_id) as "order_total"
FROM customers as cust;

我得到:

Subquery returns more than 1 row

我明白为什么会出现这个错误。因为是的，子查询确实返回多行。我想要的是将每一行的值放置在另一列中(至少这是我解释的问题)。例如:

email_address    |  order_count  |  order1_total  |  order2_total  |  etc...
============================================================================
john_doe@foo.com |      2        |     $100       |       $200     |   NULL

数据库的表如下所示:

客户(customer_id、email_address)
订单(order_id，customer_id)
order_items(item_id、order_id、item_price、discount_amount、数量)

作业中提出的问题:
(我认为措辞确实很糟糕，但我知道什么？):)

编写一条 SELECT 语句，为每个具有以下列的订单的客户返回一行:

• 客户表中的 email_address
• 订单数量统计
• 每个订单的总金额(提示:首先从价格中减去折扣金额，然后乘以数量。)

仅返回客户拥有超过 1 个订单的行。 按行项目金额的总和降序对结果集进行排序。

Answer 1

您可以通过使用 GROUP BY 聚合来避免此类关联子查询:

SELECT
    c.email_address,
    COUNT(DISTINCT o.order_id) AS num_orders,
    COALESCE(SUM(oi.quantity * (oi.item_price - oi.discount_amount)), 0) AS total_amount
FROM customers c
LEFT JOIN orders o
    ON c.customer_id = o.customer_id
INNER JOIN order_items oi
    ON o.order_id = oi.order_id
GROUP BY
    c.customer_id,
    c.email_address;