联合中的订单列由另一列选择

Question

I have a table:

Message

id, user_from_id, user_to_id, send_date, message, is_readed

and I would like to select all the users that I chat with, so my query looks like this:

SELECT user_from_id as user_id
FROM message
WHERE user_to_id=xxx
UNION
SELECT user_to_id
FROM message
WHERE user_from_id=xxx

..and I have a list of user_id's, the problem is that I can't find a way to order those id's by send_data, so that at the beginning of that list i would have a user that i lately chat with

Can someone help me?

Answer 1

How about dispensing with the union ?

SELECT (CASE WHEN m.user_to_id = xxx THEN m.user_from_id
             ELSE m.user_to_id
        END) as user_id
FROM message m
WHERE xxx IN (m.user_to_id, m.user_from_id)
ORDER BY m.send_date DESC;

This is probably not what you really want, because users will appear multiple times in the list. What you really want is:

SELECT (CASE WHEN m.user_to_id = xxx THEN m.user_from_id
             ELSE m.user_to_id
        END) as user_id
FROM message m
WHERE xxx IN (m.user_to_id, m.user_from_id)
GROUP BY m.user_id
ORDER BY MAX(m.send_date) DESC;

Answer 2

with x as(
SELECT user_from_id as user_id, send_date
FROM message
WHERE user_to_id=xxx
UNION
SELECT user_to_id, send_date
FROM message
WHERE user_from_id=xxx
) select user_id 
from x 
order by send_date

This wraps the UNION query up with the send_date column into a single variable, from which you can then SELECT and ORDER BY send_date .