Oracle SQL如何编写一个sql语句来验证我的网络中的用户(即朋友或朋友的朋友)

Question

我有这个问题.给定一个users表,其中包含用户在社交网络friends中的用户名和包含用户姓名和用户姓名的表格,如下所示...

username friendname

John        Thomas
Chris       James

...我正在尝试编写一个SQL语句,如果用户在我的网络中.换句话说,用户是朋友的朋友还是朋友？

我一直在围绕这个问题跳舞,只能提出这个问题:

SELECT f2.username, f2.friendname 
FROM friends f2 
WHERE f2.username IN (
      SELECT f1.friendname 
      FROM friends f1 
      WHERE f1.username = 'Thomas') 
AND f2.friendname <> 'user1' 
AND f2.friendname = 'user2';    

它基本上检查用户是否是我朋友的朋友,即如果为false则返回null.

试图找出我如何扩展以通过我所有的朋友网络.我的意思不仅仅是我朋友的朋友.

Answer 1

SELECT  *
FROM    (
        SELECT  username
        FROM    friends
        START WITH
                username = 'myname'
        CONNECT BY
                friendname = PRIOR username
                AND level <= 3
        )
WHERE   username = 'friendname'
        AND rownum = 1

根据需要更新级别:您可以搜索第三层朋友等.

如果友谊关系是对称的,则应进行以下查询:

WITH    q AS
        (
        SELECT  username, friendname
        FROM    friends
        UNION ALL
        SELECT  friendname, username
        FROM    friends
        ),
        f AS
        (
        SELECT  friendname, level
        FROM    q
        START WITH
                username = 'Thomas'
        CONNECT BY NOCYCLE
                username = PRIOR friendname
        )
SELECT  *
FROM    f
WHERE   friendname = 'Jo'
        AND rownum = 1

如果对表进行非规范化,则可以更快地进行此查询:每个友谊存储两条记录,如下所示:

CREATE TABLE dual_friends (orestes NOT NULL, pylades NOT NULL, CONSTRAINT pk_dualfriends_op PRIMARY KEY (orestes, pylades)) ORGANIZATION INDEX
AS
SELECT  username, friendname
FROM    friends
UNION ALL
SELECT  friendname, username
        FROM    friends

然后你可以用以下代码替换CTE上面的dual_friends:

WITH    f AS
        (
        SELECT  pylades, level
        FROM    dual_friends
        START WITH
                orestes  = 'Thomas'
        CONNECT BY NOCYCLE
                orestes = PRIOR pylades
                AND level <= 3
        )
SELECT  *
FROM    f
WHERE   pylades = 'Jo'
        AND rownum = 1

,它将使用索引并且效率更高,特别是如果您将级别限制为某个合理的值.