mySQL SELECT AS用于多个标准

Question

I need to combine these two queries into 1:

SELECT user1ID AS friendID FROM friends WHERE user2ID = '$userid'
UNION
SELECT user2ID AS friendID FROM friends WHERE user1ID = '$userid'

Then

SELECT firstName, lastName FROM users WHERE ID = friendID (that we just established)

Answer 1

I guess MySQL's UNION (Manual) keyword is what you are looking for.

EDIT due to edited question:

You will need a subquery for what you are trying to do (a JOIN might also work, but would be more complicated):

SELECT firstName, lastName FROM users WHERE ID IN
(
    SELECT user1ID AS friendID FROM friends WHERE user2ID = '$userid'
    UNION
    SELECT user2ID AS friendID FROM friends WHERE user1ID = '$userid'
)

Note that subqueries can cause performance issues in very large tables and/or complicated queries. You may want to refactor the subquery to a JOIN in that case.