mysql - 将子查询转换为连接查询

Question

对于这个查询:

SELECT `user_id`, `user_name` FROM users WHERE `user_id` IN (SELECT user_one, user_two FROM friends WHERE user_one='$my_id' OR user_two = '$my_id')

我收到此错误消息:

Cardinality violation: 1241 Operand should contain 1 column(s)

是否可以在连接中使用 2 列而不是子查询？如果是，怎么做？

Answer 1

为什么不使用这样的联合:

SELECT `user_id`, `user_name` 
        FROM users 
           WHERE `user_id` IN (SELECT user_one FROM friends 
                                               where user_one = '$my_id'
                               UNION
                               SELECT user_two FROM friends 
                                               where user_two = '$my_id')