Question

假设我有一个电子邮件功能，我需要选择电子邮件的收件人。

users表格包含以下列：id，full_name

emails表包含以下列：id，subject，body，sender_id，recipient_id

伪代码是

SELECT
  e.id AS email_id,
  e.subject AS email_subject,
  u_sender.full_name AS sender_name,
  u_recipient.full_name AS recipient_name
FROM emails e
LEFT JOIN users u_sender
  ON e.sender_id = u_sender.id
LEFT JOIN users u_recipient
  ON e.recipient_id = u_recipient.id

这将返回如下内容：

| email_id | email_subject | sender_name | recipient_name |
| 1 |你好 |约翰 |史蒂夫|
| 1 |你好 |约翰 |马克|
| 1 |你好 |约翰 |彼得|

请注意，它们具有相同的email_id，因为我的应用程序可以将电子邮件发送给多个收件人。

我找到a similar question，但解决方案是apache-exclusive。

我还发现了一个使用XML PATH here的响应，但它适用于SQL Server 2005.我正在使用postgres。

提前致谢。

Answer 1

PostgreSQL 9提供string_agg聚合函数，将多个值展平为单个字符串。在您的示例中，它将是string_agg(u_recipient.full_name, ',')以及合适的group by：

select
  e.id email_id,
  e.subject email_subject,
  u_sender.full_name sender_name,
  string_agg(u_recipient.full_name, ',') recipient_name
from emails e
left join users u_sender
  on e.sender_id = u_sender.id
left join users u_recipient
  on e.recipient_id = u_recipient.id
group by email_id, email_subject, sender_name

Answer 2

将GROUP BY子句与email_id，email_subject，sender_name一起使用，并调用STRING_AGG（recipient_name，'，'）

你会有这样的事情：

SELECT email_id, email_subject, sender_name, string_agg(recipient_name, ',') FROM test GROUP BY email_id, email_subject, sender_name

使用您的查询：

WITH X AS 
(
SELECT
  e.id AS email_id,
  e.subject AS email_subject,
  u_sender.full_name AS sender_name,
  u_recipient.full_name AS recipient_name
FROM emails e
LEFT JOIN users u_sender
  ON e.sender_id = u_sender.id
LEFT JOIN users u_recipient
  ON e.recipient_id = u_recipient.id
)
SELECT email_id, email_subject, sender_name, string_agg(recipient_name, ',') FROM X GROUP BY email_id, email_subject, sender_name;

SQL如何将行“收集”到一行

2 个答案: