How to select unique rows only in sql?

Question

I have two columns, senderUserID and recieverUserID in Messages table. I select senderUserID and recieverUserID where current receiver sent a message to current sender in the past. I select 10 rows only each time but sometimes in this table senderUserID appears more than once when i need that only senderUserID will be unique, while recieverUserID can return as many times as it happens.

this is sample data

66622   61350
90166   79222
90176   79222
86727   80452
10888   47305
66560   79219
66622   80452
89548   14452
66622   69177
52081   79223

as you can see 66622 appears twice in senderUserID. How do i limit it to appears only once.

thanks

Answer 1

I'd say just ignore any duplicates. That will give you less than ten results for each of your batches, but I'd prefer that to enforcing uniqueness via grouping on the database (could be expensive). The objective here is to maximize message throughput, right?

If you still want to do it in SQL:

 select senderUserId, max(receiverUserId) from messages group by senderUserId

Answer 2

;WITH cte AS
(
SELECT senderUserID, 
       recieverUserID,
       ROW_NUMBER() OVER (PARTITION BY senderUserID ORDER BY recieverUserID) AS RN
FROM YourTable
)
SELECT senderUserID,recieverUserID FROM cte 
WHERE RN=1