MySQL选择两列之一匹配的位置，并从另一张表中按列排序

Question

I have the following mysql statement where I select all friends for a user.

SELECT * 
FROM friends 
WHERE (user1 = FRIEND_ID_HERE AND confirmed = 1) 
   OR (user2 = FRIEND_ID_HERE AND confirmed = 1);

The problem is that the friend can be either user1 or user2 in the friends table so I dont really know how to join the members table and sort them by last_online when I dont know which column contains the friend.

How can I do this most efficiently? Thanks!

table members

user_id | username | last_online
--------------------------------
      1 |    user1 | 1502296162
      2 |    user2 | 1502296161
      3 |    user3 | 1502296160
      4 |    user3 | 1502296159

table friends

 id | user1 | user2 | confirmed
-------------------------------
  1 |     1 |     2 | 1502296162
  2 |     3 |     1 | 1502296161
  3 |     2 |     4 | 1502296160

Answer 1

I think what you need is something like this

SELECT * FROM members JOIN (
  SELECT user2 AS id FROM friends WHERE (user1 = FRIEND_ID_HERE AND confirmed = 1)
  UNION
  SELECT user1 AS id FROM friends WHERE (user2 = FRIEND_ID_HERE AND confirmed = 1)
) f ON memebers.id = f.id
ORDER BY last_online;

Answer 2

SELECT * FROM friends WHERE confirmed=1 AND friend_id_here IN (user1,user2)