Question

我在sql中有一个消息表，它基本上是当前用户的列，他正在与之交互的用户，id和消息的内容。我需要从每个不同的对话中检索最后一条消息，以便进行预览。

任何想法如何？

Answer 1

使用NOT EXISTS读取该会话中没有后续消息的所有行。类似的东西：

select *
from messages m1
where not exists (select 1 from messages m2
                  where m2.messageid > m1.messageid
                    and m1.conversationid = m2.conversationid)

示例输出：

SQL>create table messages (conversationid int, messageid int);
SQL>insert into messages values (1,1);
SQL>insert into messages values (1,2);
SQL>insert into messages values (2,1);
SQL>insert into messages values (2,2);
SQL>insert into messages values (2,3);
SQL>insert into messages values (3,1);
SQL>select *
SQL&from messages m1
SQL&where not exists (select 1 from messages m2
SQL&                  where m2.messageid > m1.messageid
SQL&                    and m1.conversationid = m2.conversationid);
conversationid   messageid
============== ===========
             1           2
             2           3
             3           1

                  3 rows found

Answer 2

每组最大问题通常使用modern SQL使用窗口函数来解决：

select *
from (
  select c.*,
         row_number() over (partition by conversation_id order by message_time desc) as rn
  from conversation c
) t
where rn = 1
order by conversation_id;

从每个对话sql获取最后一条消息

2 个答案: