从 MySQL 中分组和连接表的特定行中选择数据

Question

我有两个表，客户和订单，它们是内部连接的。一个客户可以有多个与其关联的订单。在我的选择中，我然后按customers.id 分组。我需要选择每个客户最近的订单，还要选择在该订单上花费的金额。目前，我可以选择最近的order_date，但不知道如何选择与order_date在同一行的金额。

这是我当前的查询：

SELECT 
    first_name, 
    last_name, 
    email, 
    MAX(order_date) AS recent_order, 
    amount -- this needs to select amount associated with recent_order
FROM customers
JOIN orders
    ON customers.id = orders.customer_id
GROUP BY customers.id;

查询选择最近的日期，但不选择与最近的order_date关联的金额。

表声明：

CREATE TABLE customers (
    id INT AUTO_INCREMENT PRIMARY KEY,
    first_name VARCHAR(100),
    last_name VARCHAR(100),
    email VARCHAR(100)
);

CREATE TABLE orders (
    id INT AUTO_INCREMENT PRIMARY KEY,
    order_date DATE,
    amount DECIMAL(8,2),
    customer_id INT,
    FOREIGN KEY(customer_id) REFERENCES customers(id)
);

Answer 1

我建议在where 子句中使用相关子查询：

SELECT c.*, o.*  -- or whatever columns you want
FROM customers c JOIN
     orders o
     ON c.id = o.customer_id
WHERE o.order_date = (SELECT max(o2.order_date)
                      FROM orders o2
                      WHERE o2.customer_id = o.customer_id
                     );

为了提高性能，您需要在orders(customer_id, order_date) 上建立索引。

Answer 2

使用这个：

SELECT TOP 1
    t1.first_name, 
    t1.last_name, 
    t1.email, 
    t2.order_date,
    t2.amount
FROM customers t1
JOIN orders t2
    ON t1.id = t2.customer_id
ORDER BY
    t2.order_date

如果意图返回所有具有最近 order_date 的行，请添加 GROUP BY t1.id。如果每个订单都被orders 中的一行重新发送，请忽略此选项。请注意，这不会合计金额。您必须在代码中执行此操作，否则使用不同的查询。另请注意，此查询的性能将受索引配置的影响。如果 order_date 不是索引的一部分并且表包含大型数据集，则此查询可能无法执行。

Answer 3

SELECT 
    first_name, 
    last_name, 
    email, 
    MAX(order_date) AS recent_order, 
    (SELECT amount FROM orders WHERE order_date = MAX(order_date) AND customers.id = orders.customer_id) as amount
FROM customers
JOIN orders
    ON customers.id = orders.customer_id
GROUP BY customers.id;

或

SELECT 
    first_name, 
    last_name, 
    email, 
    order_date AS recent_order, 
    amount AS recent_order_amount
FROM customers
JOIN orders
    ON customers.id = orders.customer_id

GROUP BY customers.id
ORDER BY orders.order_date DESC;