Question

如何创建联接以获取所有客户的最新发票？

Tables:
- Invoices
- Customers

Customers table has: id, last_invoice_sent_at, last_invoice_guid
Invoices table has: id, customer_id, sent_at, guid

我想为每位客户提取最新发票，并使用该数据更新Customers表中的last_invoice_sent_at和last_invoice_guid。

Answer 1

您想使用distinct on。对于按customer_id然后按invoice查询的查询，它将返回distinct on中指示的每个不同值的第一行。这是下面有*的行：

customer_id | sent_at     |
1           | 2014-07-12  | * 
1           | 2014-07-10  | 
1           | 2014-07-09  |
2           | 2014-07-11  | *
2           | 2014-07-10  |

因此，您的更新查询可能如下所示：

update customers
set last_invoice_sent_at = sent_at
from (
  select distinct on (customer_id)
    customer_id,
    sent_at
  from invoices
  order by customer_id, sent_at desc
) sub
where sub.customer_id = customers.customer_id

Answer 2

@Konrad提供了一个完美的SQL语句。但由于我们只对单个列感兴趣，因此GROUP BY将比DISTINCT ON更有效（这对于从同一行检索多个列非常有用）：

UPDATE customers c
SET    last_invoice_sent_at = sub.last_sent
FROM  (
   SELECT customer_id, max(sent_at) AS last_sent
   FROM   invoices
   GROUP  BY 1
   ) sub
WHERE sub.customer_id = c.customer_id;

在SQL中获取子查询的结果

2 个答案: