Mysql statement to select distinct rows from two columns

问题

I'm trying to write a mysql statement that pulls only unique messages from my chat system. There is a to_id and a from_id. I want to pull one row from every conversation.

The problem is if there is a row with to_id = 1 from_id = 2 and another row with to_id = 2 from_id = 1 I want both of these rows to be treated as the same.

The statement I came up with is "SELECT * FROM chat_messages GROUP BY to_id,from_id ". This works except for the situation I mentioned above.

Another attempt I made was using "SELECT DISTINCT least(to_id,from_id) as value1, greatest(to_id,from_id) as value2 from chat_messages ".

This returns what I need but I do not know how to get the rest of the information from the row like the message, timestamp etc. It only gives me value1 & value2 which are the to_id & from_id.

回答1:

Just use the expressions in the second query for the GROUP BY of the first query:

SELECT * FROM chat_messages GROUP BY LEAST(to_id,from_id), GREATEST(to_id,from_id)

回答2:

If second query returns what you expect, then below query returns rest information:

SELECT *
FROM chat_messages t1 INNER JOIN (
    SELECT DISTINCT LEAST(to_id, from_id) AS value1,
        GREATEST(to_id, from_id) AS value2
    FROM chat_messages
) t2 ON t1.to_id = value1 AND t1.from_id = value2;

来源：https://stackoverflow.com/questions/26707468/mysql-statement-to-select-distinct-rows-from-two-columns