Question

我有这个SQL（SQLite数据库），它正确地从messages_id分组的消息表中获取最新消息，其中消息的会话ID不是'none'。

SELECT * from messages 
WHERE _id
IN ( 
   SELECT MAX(_id)
   FROM messages
   WHERE conversation_id != 'none'
   GROUP BY conversation_id
   )

但是，我想在对话表中使用“unseenreplies”列（其中包含对此会话的看不见的回复的数量），以订购出来的消息。< / p>

我试过这个：

SELECT * from messages
WHERE _id IN 
(
   SELECT max(_id) from messages m
   JOIN conversations c 
   ON m.conversation_id = c.cid ORDER BY 
   c.unseenreplies DESC 
   where m.conversation_id != 'none' group by m.conversation_id )

但是我在'where'子句附近遇到语法错误。

仅作为解释，'conversation id'不是会话表的主键，而是标识符字符串。

我该如何解决这个问题？我的方法是否以JOIN方式关闭？

Answer 1

JOIN代替IN谓词：

SELECT * 
from messages AS m1
INNER JOIN conversations c1 ON m1.conversation_id = c1.cid
INNER JOIN 
(
   SELECT max(_id) AS MAxId 
   from messages m
   where m.conversation_id != 'none' 
   GROUP BY m.conversation_id 
) AS m2 ON m1._id = m2.MaxId
ORDER BY  c.unseenreplies DESC

Answer 2

您无需删除in语句即可使其生效。关键是conversations和messages之间的联接：

SELECT m.*
from messages m join
     conversations c
     on m.conversation_id = c.cid
WHERE _id
IN ( 
   SELECT MAX(_id)
   FROM messages
   WHERE conversation_id != 'none'
   GROUP BY conversation_id
   ) 
order by c.unseenreplies desc

另外，请注意，这只会从messages表而不是所有列中提取列。

SQL：在另一个表中按列desc排序

2 个答案: