SQL / Laravel：是否可以基于两个列值返回重复数据删除结果？

Question

I have a fairly standard Chat model, which contains messages. For simplicity sake, let's say it has three columns: to_user_id , from_user_id and message .

I want to return a deduped results based on the two column values: to_user_id and from_user_id . This is a some I can list the ongoing conversations. I can return a return a deduped set of results using one column value. However, this essential comes back like the below. Obviously, this is because the user can be both the to_user_id and from_user_id depending on whether they sent or received the message.

Tom > Harry
Harry > Tom
John > Chris
Chris > John

I am hoping to get is something like the below, so it groupBy

Tom > Harry
John > Chris

Conversations are one-to-one, so will only ever have two participants.

Any help would be greatly appreciated!

Answer 1

Not the best solution, but I suppose it should work. So idea in sort by user_id, so you will get unique pairs of conversation.

Select if(to_user_id > from_user_id, concat(from_user_id, ':', to_user_id), concat(to_user_id, ':', from_user_id)) as pair from messages group by pair;

Answer 2

This might work:

SELECT DISTINCT
    if(to_user_id > from_user_id,to_user_id ,from_user_id) as source,  
    if(to_user_id > from_user_id,from_user_id,to_user_id ) as target 
FROM conversations;

Probably not suitable to be part of anything more complex.

Answer 3

I managed to join this up with data from the users table with the below solution. Thought I would share.

Select O.Sender, S.First_Name, S.Last_Name, O.Target, T.First_Name, 
T.LastName
From (
SELECT DISTINCT
    if(to_user_id > from_user_id, to_user_id, from_user_id) as sender,
    if(to_user_id > from_user_id, from_user_id, to_user_id ) as target 
FROM chats AS pair
) O
LEFT JOIN Users S ON O.Sender = S.UserID
LEFT JOIN Users T ON O.Target = T.UserID