Question

我的数据库中有这个表（关注者）。

follower | following
---------|----------
1        |2
1        |4
1        |5
1        |6
1        |7
1        |8
2        |4
4        |6

此文档记录了我的用户的所有以下内容和关注者。我有这个查询，这是无效的。

SELECT following 
FROM followers 
WHERE follower 
IN (SELECT following 
    FROM followers 
    WHERE follower = 2) 
OR following IN (SELECT follower 
                 FROM followers 
                 WHERE following = 2)

我想要实现的是用户列表，其后跟用户“2”跟随的用户，与用户“2”后面的用户列表合并。排除所有已经被用户“2”跟随的人。并通过它们与用户“2”的密切关系来安排。

理想情况下，我的数据输出是这个（但不是自动的顺序）：

suggested_users
---------------
6
1
5
7
8

我实际上对如何提出这个问题感到茫然，但我如何得到这些结果呢？

Answer 1

如果你可以选择转储mysql，一些sql变种可以让你使用递归查询来获取树：

http://www.postgresql.org/docs/current/static/queries-with.html

如果没有，有各种解决方案;通常是嵌套集或嵌套间隔：

http://en.wikipedia.org/wiki/Nested_set_model

http://en.wikipedia.org/wiki/Nested_intervals

（SO上有很多关于两者的问题。）

Postgres的声明是什么？我可能会切换，MySQL似乎有点软：P

未经测试，但它类似于：

with recursive tree (root, node) as (
select following as root, follower as node
  from followers
union all
select tree.root, followers.follower
  from followers
  join tree on tree.node = followers.following
)
select * from tree where root = 2;

优化版本如下：

with recursive tree (root, node) as (
select following as root, follower as node
  from followers
 where following = 2
union all
select tree.root, followers.follower
  from followers
  join tree on tree.node = followers.following
)
select * from tree;

Answer 2

这可以通过加入followers表格来完成。

试试这个：

SELECT f2.following suggested_users
FROM followers f1
INNER JOIN followers f2 ON f2.follower = f1.following
WHERE f1.follower = 2
GROUP BY f2.following

UNION

SELECT f3.follower suggested_users
FROM followers f3
WHERE f3.following = 2
GROUP BY f3.follower

此查询的一般概念是：

f1.follower跟随f1.following和f1.following = f2.follower
f2.follower跟随f2.following

结合

f3.follower跟在f3.following和f3.following = 2

表连接将如下所示：

第一个子查询

f1.follower | f1.following = f2.follower | f2.following | suggested_users |
------------|----------------------------|--------------|-----------------|
2           |4                           |6             | 6               |

第二个子查询

f3.follower | f3.following | suggested_users |
------------|--------------|-----------------|
1           |2             |1                |

在MySQL中选择分层数据

2 个答案: