从MySQL中分组和联接表的特定行中选择数据

Question

我有两张桌子，客户和订单是内部连接的。客户可以有多个与其关联的订单。在我的选择中，我然后按客户分组。id.我需要选择每个客户的最新订单，还需要选择在该订单中花费的金额。目前，我可以选择最近的订单日期，但不知道如何选择与订单日期同一行中的金额。

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;

查询选择最近日期，但不选择与最近订单日期关联的金额。

表格声明：

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

使用以下命令：

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

通过t1添加一个分组。id如果目的是返回包含最近订单日期的所有行。如果每个订单都由订单中的一行重新提交，请忽略此项。请注意，这不会合计金额。您必须在代码中这样做，否则将使用不同的查询。还要注意，此查询的性能将受到索引配置的影响。如果order_date不是索引的一部分，并且该表包含大型数据集，则可能无法执行此查询。

Answer 2

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;

或者

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;

Answer 3

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，您需要一个关于订单（客户id、订单日期）的索引。