sparql：为每个节点随机选择一个连接

Question

我有以下数据：

<node:1><urn:connectTo><node:2>
<node:1><urn:connectTo><node:3>
<node:1><urn:connectTo><node:4>
<node:2><urn:connectTo><node:10>
<node:2><urn:connectTo><node:11>
<node:2><urn:connectTo><node:12>
<node:3><urn:connectTo><node:21>
<node:3><urn:connectTo><node:13>
<node:3><urn:connectTo><node:41>
<node:3><urn:connectTo><node:100>
<node:4><urn:connectTo><node:119>
<node:4><urn:connectTo><node:120>

如您所见，每个节点都有多个连接。我想为每个节点随机选择一个连接。我怎样才能做到这一点？我尝试了以下查询，但都没有解决问题：

1. select ?currentNode ?nextNode where {
     ?currentNode ?p ?nextNode
     BIND(RAND() AS ?orderKey)
   }
   ORDER BY ?orderKey
   LIMIT 1
   

2. select ?currentNode SAMPLE(?nextNode) as ?nextNode1
   where {
     ?currentNode ?p ?nextNode
   }
   GROUP BY ?currentNode
   

   注意：结果给出了每个节点的第一个连接，但不是随机的

3. select ?currentNode ?nextNode (COUNT(?nextNode) AS ?noOfChoices)
   where {
     ?currentNode ?p ?nextNode
     BIND(RAND() AS ?orderKey)
   }
   GROUP BY ?currentNode
   ORDER BY ?orderKey
   OFFSET (RAND()*?noOfChoices)
   LIMIT 1
   

Answer 1

sample aggregate 返回组内的个人：

Sample 是一个集合函数，它从 多集传递给它。 … 例如，给定 Sample({"a", "b", "c"})、"a"、"b" 和 "c" 都是有效的返回值。注意 Sample() 不需要对于给定的输入是确定性的， 唯一的限制是输出值必须存在于输入中 多集。

这将是一个类似的查询：

prefix node: <node:>
prefix urn: <urn:>

select ?source (sample(?_target) as ?target) where {
  ?source urn:connectTo ?_target
}
group by ?source

---------------------
| source | target   |
=====================
| node:1 | node:2   |
| node:2 | node:10  |
| node:3 | node:13  |
| node:4 | node:119 |
---------------------

当然，正如您所注意到的，实现只需要返回一个任意个体。这很容易每次都相同。您可以在子查询中进行一些排序，并希望随机化目标的顺序以便从 sample 中获得不同的结果，但不要求结果的顺序来自子查询也被保留。看起来像这样：

prefix node: <node:>
prefix urn: <urn:>

select ?source (sample(?_target) as ?target) where {
  { select ?source ?_target {
      ?source urn:connectTo ?_target
    }
    order by rand() }
}
group by ?source

这似乎适用于 Apache Jena。以下是重复调用的结果：

---------------------
| source | target   |
=====================
| node:1 | node:2   |
| node:2 | node:11  |
| node:3 | node:100 |
| node:4 | node:120 |
---------------------

---------------------
| source | target   |
=====================
| node:1 | node:3   |
| node:2 | node:11  |
| node:3 | node:13  |
| node:4 | node:120 |
---------------------

---------------------
| source | target   |
=====================
| node:1 | node:3   |
| node:2 | node:10  |
| node:3 | node:21  |
| node:4 | node:119 |
---------------------

---------------------
| source | target   |
=====================
| node:1 | node:3   |
| node:2 | node:10  |
| node:3 | node:100 |
| node:4 | node:119 |
---------------------