mysql：从子查询中获取两个值

Question

我正在尝试使用子查询来检索每个客户的最早订单。我要选择email_address、order_id和order_date

表格：

客户（customer_id、email_address）

订单（order_id、order_date、customer_id）

我尝试过的：

我可以通过这样做获得order_id 或order_date

SELECT email_address, 
       (SELECT order_date /* or order_id */
        FROM orders o
        WHERE o.customer_id = c.customer_id
        ORDER BY order_date LIMIT 1)
FROM customers c
GROUP BY email_address;

但如果我尝试在子查询中执行 SELECT order_id, order_date，则会收到错误消息：

操作数应包含 1 列

Answer 1

您可以使用 JOIN 解决此问题，但您需要注意仅将 JOIN 设置为给定客户的最旧值：

SELECT c.email_address, o.order_id, o.order_date
FROM customers c
JOIN orders o ON o.customer_id = c.customer_id AND
    o.order_date = (SELECT MIN(order_date) FROM orders o2 WHERE o2.customer_id = c.customer_id)

Answer 2

您可以使用 JOIN 来获得您想要的结果，或者修改您的查询如下：

SELECT email_address, 
    (SELECT order_date
    FROM orders o1
    WHERE o1.customer_id = c.customer_id
    ORDER BY order_date LIMIT 1) as `order_date`,
    (SELECT order_id
    FROM orders o2
    WHERE o2.customer_id = c.customer_id
    ORDER BY order_date LIMIT 1) as `order_id`
FROM customers c
GROUP BY email_address;

Answer 3

JOIN 由您选择。 How can I select multiple columns from a subquery (in SQL Server) that should have one record (select top 1) for each record in the main query?

SELECT o.order_id, c.email_address, o.order_date
FROM customers c
INNER JOIN (
    SELECT order_date, order_id, customer_id
    FROM orders o
    ORDER BY order_date
) as o on o.customer_id = c.customer_id
GROUP BY email_address;