java - JPA，playframework，在分组依据之前选择顺序

Question

如何在“JPA”中编写查询会在 GROUP BY 之前进行 ORDER BY 吗？

例如:

select m from Message m where m.id = (
  select distinct m2.conversation_id
    from Message m2
   where m2.from = 100 or m2.to = 100
)
group by m.conversation_id order by m.date desc

我想要每个对话 ID 的最后(最新)消息。

Answer 1

简单来说，不能将 ORDER BY 放在 GROUP BY 之前； SQL 需要在 GROUP BY 之后添加 ORDER BY。

但是，您可以实现所需的结果 - 使用下面的冗长方法或使用不同的 ORDER BY 子句构建查询:

ORDER BY m.conversation_id, m.date DESC

在从数据库获取数据的代码中，您为返回的每个新对话 ID 选择第一行。

一如既往，我分阶段构建查询。首先，查找涉及“用户 ID 100”的每个对话 ID 的最新消息时间戳:

SELECT m2.conversation_id, MAX(m2.date) AS date
  FROM Message AS m2
 WHERE m2.from = 100 OR m2.to = 100
 GROUP BY m2.conversation_id

现在，将其与原始表连接起来以获得感兴趣的行:

SELECT m1.*
  FROM Message AS m1
  JOIN (SELECT m2.conversation_id, MAX(m2.date) AS date
          FROM Message AS m2
         WHERE m2.from = 100 OR m2.to = 100
         GROUP BY m2.conversation_id
       ) AS m3
    ON m1.conversation_id = m3.conversation_id AND m1.date = m3.date
 ORDER BY m1.date DESC;