SQL获取用户朋友和朋友的朋友

Question

我的MySQL表结构是这样的.

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

1,2
2,1

我想得到特定用户朋友的朋友和朋友.

select应该返回列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

然后查询获得1的朋友和朋友的朋友应该返回:

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

我可以使用单个查询获取此行吗？

更新答案:我稍微修改了DVK的答案,这是返回我正在寻找的内容的查询.

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;

Answer 1

这是效率较低但可读的：

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

或者

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