postgresql：加入第n行

Question

我有一个表 users 和一个表 orders。

表 users 有一个主键 user_id 和一些其他字段。表orders有一个外键user_id，一个名字order_name和一个日期order_date。

对于旧版导出，我需要构建一个查询以返回表单中的数据 user_id | order_name1 | order_name2 | order_name3 其中订单 1-3 是客户最近的三个订单。

我的想法是查询客户并使用 select to orders 进行三个连接

SELECT 
 users.*,
 order1.order_name AS order_name1,
 order2.order_name AS order_name2,
 order3.order_name AS order_name3
 FROM users AS users
JOIN (
 SELECT
  user_id,
  order_name,
  order_date
 FROM orders ORDER BY order_date DESC OFFSET 0 LIMIT 1)
AS order1 ON order1.user_id=users.user_id
JOIN (
 SELECT
  user_id,
  order_name,
  order_date
 FROM orders ORDER BY order_date DESC OFFSET 1 LIMIT 1)
AS order2 ON order2.user_id=users.user_id
JOIN (
 SELECT
  user_id,
  order_name,
  order_date
 FROM orders ORDER BY order_date DESC OFFSET 2 LIMIT 1)
AS order3 ON order3.user_id=users.user_id

但是，这不起作用，因为它只从订单中返回一行，而不是专门针对每个用户。

这样的查询怎么写？

Answer 1

您可以使用window function 为每个用户的行编号：

SELECT users.*,
       o.order_name,
       o.rn as order_number
FROM users AS users
JOIN ( SELECT user_id,
              order_name,
              order_date, 
              row_number() over (Partition by user_id) order by order_date desc as rn
       FROM orders
) AS o ON o.user_id = users.user_id and o.rn <= 3;

但是，以上内容不会为您提供三列，而是为每个用户提供三行。要将其转换为列，您需要通过应用 group by 来应用穷人的枢轴模式：

SELECT users.user_id,
       max(case when o.rn = 1 then o.order_name end) as order_name1,
       max(case when o.rn = 2 then o.order_name end) as order_name2,
       max(case when o.rn = 3 then o.order_name end) as order_name3
FROM users AS users
JOIN ( SELECT user_id,
              order_name,
              order_date, 
              row_number() over (Partition by user_id) order by order_date desc as rn
       FROM orders
) AS o ON o.user_id = users.user_id and o.rn <= 3;
GROUP BY users.user_id;

如果users.user_id 是该表的主键，您可以从users 表中添加其他列，而无需将它们添加到group by。

从 Postgres 9.4 开始，表达式

max(case when o.rn = 1 then o.order_name end) as order_name1

也可以写成：

max(o.order_name) filter (where o.rn = 1) as order_name1