如何按计数限制 SQL LEFT JOIN 查询的结果集

Question

我希望使用 LEFT JOIN 查询从具有一对多关系的两个表中获取一些结果，但根据“孩子”的数量限制结果集。我有两个表结构如下：

customers
id    name    ...
1     "bob"   ...
2     "jill"  ...

orders
id    customer_id    ...
100   1              ...
101   2              ...
102   1              ...

（表中的其余数据与此查询无关。）

我想做的是获取所有客户 ID 及其订单 ID，按客户排序，但仅限于下过多个订单的客户。在此示例中，结果将看起来像：

cust_id    order_id
1          100
1          102

我已经开始使用基本的 LEFT JOIN 来将订单 ID 与他们的客户配对，但不知道如何排除所有未订购至少两次的客户。

SELECT 
    `customers`.`id` AS `cust_id`,
    `orders`.`id` AS `order_id`
FROM
    `customers`
LEFT JOIN `orders` ON
    `customers`.`id` = `orders`.`customer_id`
ORDER BY
    `cust_id`

谢谢大家。

Answer 1

一种方法是创建一个内联视图，以获取拥有多个订单的客户及其内部连接。如果您不喜欢 JOIN，您也可以执行 IN 或 EXISTS

SELECT 
    `customers`.`id` AS `cust_id`,
    `orders`.`id` AS `order_id`
FROM
    `customers`
LEFT JOIN `orders` ON
    `customers`.`id` = `orders`.`customer_id`
INNER JOIN 
      (SELECT `customer_id `    
       FROM `orders`
       GROUP BY `customer_id`    
       HAVING COUNT(id) > 1) as `morethanone`
On
    `customer`.`id`  = `morethanone`.`custmor_id`
ORDER BY
    `cust_id`

Answer 2

嘿，我想我有点慢，但无论如何我都会发布我的答案。它略有不同，但仍然有效。

SELECT `customers`.`id` AS `cust_id` , count( orders.id ) AS count_orders
FROM `customers`
JOIN `orders` ON `customers`.`id` = `orders`.`customer_id`
GROUP BY customers.id
HAVING count( orders.id ) >1
ORDER BY `cust_id`

Answer 3

首先，您不需要将联接保留在外部，因为它与内部联接之间的唯一区别是包括没有订单的客户，并且您不需要在结果中使用他们，因为他们的订单较少超过两个订单。

SELECT customers.id AS `cust_id`, orders.id AS `order_id`
FROM customers
     INNER JOIN orders ON `customers`.`id` = `orders`.`customer_id`
     INNER JOIN orders count_orders ON customers.id = count_orders.customer_id
GROUP BY count_orders.id, orders.id
HAVING count(count_orders.id) >= 2
ORDER BY `cust_id`

Answer 4

如果您排除了尚未订购的客户，那么 LEFT JOIN 是不必要的，不是吗？

由于您需要结果集中的订单 ID，因此需要首先运行 COUNT 查询。

在 SQL Server 2005+ 中，CTE 将完成计数查询，然后可以根据该计数查询生成您想要的结果集：

WITH customerWithMoreThanOneOrder AS
(
    SELECT 
        `customers`.`id` AS `cust_id`
    FROM
        `customers`
    INNER JOIN `orders` ON
        `customers`.`id` = `orders`.`customer_id`
    GROUP BY 
        `customers`.`id`
    HAVING COUNT(0) > 1
)

SELECT 
    `customers`.`id` AS `cust_id`,
    `orders`.`id` AS `order_id`
FROM
    `customers`
INNER JOIN `orders` ON
    `customers`.`id` = `orders`.`customer_id`
WHERE `customers`.`id` IN (SELECT cust_id FROM customerWithMoreThanOneOrder)
ORDER BY
    `cust_id`

如果您使用的数据库不支持这种结构，那么只需将计数查询放在 IN 子查询的括号中，如下所示：

SELECT 
    `customers`.`id` AS `cust_id`,
    `orders`.`id` AS `order_id`
FROM
    `customers`
INNER JOIN `orders` ON
    `customers`.`id` = `orders`.`customer_id`
WHERE `customers`.`id` IN (SELECT 
                               `customers`.`id` AS `cust_id`
                           FROM
                               `customers`
                           INNER JOIN `orders` ON
                               `customers`.`id` = `orders`.`customer_id`
                           GROUP BY 
                               `customers`.`id`
                           HAVING COUNT(0) > 1)
ORDER BY
    `cust_id`

如果您有任何问题，请告诉我

Answer 5

其他答案的替代方案，您可以使用 IN 子句。

SELECT 
    `customers`.`id` AS `cust_id`,
    `orders`.`id` AS `order_id`
FROM
    `customers`
LEFT JOIN `orders` ON
    `customers`.`id` = `orders`.`customer_id`
WHERE `customer_id` IN (SELECT `customer_id` FROM `orders` GROUP BY `customer_id` HAVING COUNT(*) > 1)
ORDER BY
    `cust_id`