SQL获取用户的朋友和朋友的朋友 [英] SQL to get friends AND friends of friends of a user

问题描述

我的MySQL表结构是这样的.

My MySQL tables structure is like this.

USER
int id
varchar username

FRIEND_LIST
int user_id
int friend_id

对于每个朋友关系，我在FRIEND_LIST中插入2条记录. 如果用户1是用户2的朋友，则将下一行插入FRIEND_LIST

For each friend relationship I insert 2 records in FRIEND_LIST. If user 1 is friend of user 2 then the next rows are inserted into FRIEND_LIST

1,2
2,1

我想结识特定用户的朋友和朋友.

I want to get the friends and friends of friends of an specific user.

该选择应返回a，b，c列.

The select should return columns a, b, c.

a: user_id
b: friend_id
c: username (username of friend_id )

If 1 is friend of 2 and 3.
2 is friend of 3, 4 and 5
3 is friend of 5,6,7

然后获取1个朋友的朋友和朋友的朋友的查询应返回:

Then the query to get 1's friends and friends of friends should return:

1 2 two
1 3 three
2 1 one
2 3 three
2 4 four
2 5 five
3 1 one
3 5 five
3 6 six
3 7 seven

我可以通过一次查询获得这些行吗?

Can I get this rows with a single query?

更新答案::我稍微修改了DVK的答案，这是返回我所要查找内容的查询.

UPDATE ANSWER: I modified DVK's answer a little bit and this is the query that returns what I was looking for.

SELECT friends.user_id, friends.friend_id, username

FROM
      FRIEND_LIST friends, USER

WHERE
      CAT_USER.id = friends.friend_id
AND
      friends.user_id = 1

UNION

SELECT
        fof.user_id, fof.friend_id, username
FROM
        FRIEND_LIST friends, FRIEND_LIST fof, USER
WHERE
        USER.id = fof.friend_id
 AND
        friends.friend_id = fof.user_id
 AND
        friends.user_id = 1;

推荐答案

效率较低，但可读性强:

This is less efficient but readable:

SELECT friends.user_id, friends.friend_id, username 
FROM FRIEND_LIST friends, USER
WHERE USER.id           = friends.friend_id
 AND  USER.id = 1
UNION

SELECT USER.user_id, fof.friend_id, username 
FROM FRIEND_LIST friends, FRIEND_LIST fof, USER
WHERE USER.id           = fof.friend_id
 AND  friends.friend_id = fof.user_id
 AND  USER.id = 1

OR

SELECT user_id, f_fof.friend_id, username 
FROM USER, (
    SELECT f.user_id, f.friend_id
    FROM   FRIEND_LIST f
    WHERE  user_id = 1
    UNION
    SELECT f.user_id, fof.friend_id
    FROM   FRIEND_LIST f, FRIEND_LIST fof
    WHERE  user_id = 1
     AND   f.friend_id = fof.user_id
) as f_fof
WHERE USER.id           = f_fof.friend_id

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