MYSQL选择往复数据? [英] MYSQL Selecting reciprocating data?

问题描述

我有一个名为Follow的表，其中包含三个字段:

I have a table called Follow, with three fields:

• Id(自动增量整数)
• UserId(int)，
• 关注(int)

如果我有这样的数据:

ID    UserId    Following   
--------------------------
1       2          3
2       3          2
3       2          5
4       2          6
5       3          5

我将如何找到用户2的朋友(即:用户2正在关注他们，而他们又跟随了用户2)

How would I find user 2's friends (ie: user 2 is following them, and they follow user 2)

我想换句话说，如果用户"a"跟随用户"b"，而用户"b"跟随用户"a"，我该如何选择用户A?

I guess, in other words, if user 'a' follows user 'b', and user 'b' follows user 'a', how do I select user A ??

推荐答案

尝试一下:

SELECT a.UserId, a.Following
  FROM Follow a INNER JOIN Follow b
    ON a.UserId = b.Following
   AND b.UserId = a.Following

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