在消息数据库表中获取不同的用户

Question

For a messages table with the following structure

id, sender_id, receiver_id, text

I want to build a list of distinct users (ordered by message id) who have either sent a message to or received one from the current session holder. The list should contain a user only once.

A query such as:

"SELECT * FROM messages WHERE sender_id = '$sess_id' OR receiver_id = '$sess_id'";

yields duplicates i.e. multiple rows with the same user. Does one necessarily have to do the filtering after the SQL query or is there an SQL query that will yield the desired unique list?

Thanks!

Answer 1

You could do:

SELECT sender_id
FROM   messages
WHERE  receiver_id = '$sess_id'
UNION
SELECT receiver_id
FROM   messages
WHERE  sender_id = '$sess_id'