如何 select 全部从一个表中按最后创建的时间戳从另一个表中的值排序

Question

I have 2 tables called conversation and message .

I would like to select all conversations sorted by order of the last updated message related to the conversation. The conversation table doesn't have the updated_at and to be able to get all conversations ordered by the last updated_at, that should be deducted from the last created message in the conversation from the table message.

The result should be based on what is the last message of the conversation.

The SQL I tried is not giving me the right result and I believe I should use a join but as I'm new to SQL cannot really get it how

select
    *
from
    conversation
inner join message on
    conversation.id = message.conversation_id
order by
    message.updated_at desc;    

Tables looks as follow

The conversation

id	topic	origin_type	origin_id	created_at
1	t1	doc1	xx1	2021-04-07 13:23:40
2	t2	doc2	xx2	2021-04-07 14:23:40
3	t3	doc3	xx3	2021-04-07 15:23:40

Message table

id	conversation_id	message	created_at	updated_at
1	1	xxx	2021-04-07 13:23:40	2021-04-07 13:23:40
2	2	xxx	2021-04-07 14:23:40	2021-04-07 14:23:40
3	3	xxx	2021-04-07 15:23:40	2021-04-07 15:23:40

Seeing the tables I should get a result of the select as follow I believe as the conversation.id = 3 has the last created and most recent message.

id	conversation_id
1	3
2	2
3	1

In SQL/PSQL I don't know how to do this kind of selection

Answer 1

Your data is not particularly illustrative, because the conversations only have a single message. However, I think you want to keep all rows for a conversation together. That would use a window function to get the maximum updated time for each conversation:

order by max(message.updated_at) over (partition by conversation.id) desc,
         conversation.id,
         message.updated_at desc

EDIT:

If you just want the conversations with no message information, you can use a subquery:

select c.*
from conversations c
order by (select max(m.updated_at) from messages m where m.conversation_id = c.id) desc;

Answer 2

You can just use a scalar subquery:

select *,
    (
        select max(m.updated_at) from message m
        where m.conversation_id = c.conversation_id
    ) as last_updated_at
from conversation c
order by last_updated_at desc;