Neo4J：返回给定键的每个值的最新数据节点

如何解决Neo4J：返回给定键的每个值的最新数据节点

我正在处理Neo4J中的数据集，该数据集会定期更新，我想编写一个查询，该查询将为我获取数据库中给定键的每个值的最新数据节点，但是我不确定如何去做。

以下是我的数据外观（简化）的示例：

[
  {
    "timestamp": "<timestamp - 3 days ago>","component": "a","value": "1"
  },{
    "timestamp": "<timestamp - 1 day ago>","value": "2"
  },{
    "timestamp": "<timestamp - 6 days ago>","component": "b","value": "5"
  },{
    "timestamp": "<timestamp - 4 days ago>","value": "9"
  },{
    "timestamp": "<timestamp - 2 days ago>","value": "11"
  },...
}

从此示例数据中，我想为每个唯一的component返回一行，并且它将是基于时间戳的最新记录：

[
  {
    "timestamp": "<timestamp - 1 day ago>",...
]

或者如果我只返回我关心的特定列：

| component | value |
| ---       | ---   |
| a         | 2     |
| b         | 11    |
| ...       | ...   |

使用cypher可以做到这一点吗？

解决方法

假设您将数据放入参数中，就可以这样做

// back to rows
UNWIND $data AS row

// order by timestamp
WITH row.component AS component,row.timestamp AS timestamp,row.value AS value
ORDER BY timestamp DESC

// get first item in the sorted collection
WITH component,COLLECT({timestamp:timestamp,value:value})[0] AS tsval

// construct the array
RETURN COLLECT({timestamp:tsval.timestamp,component:component,value:tsval.value}) AS result

或者只是

RETURN component,tsval.value AS value 

,

假设列表以data parameter的形式传递给查询，则此查询使用aggregating function COLLECT：

UNWIND $data AS d
WITH d
ORDER BY d.timestamp DESC
RETURN d.component AS component,COLLECT(d.value)[0] AS value

产生如下结果：

╒═══════════╤═══════╕
│"component"│"value"│
╞═══════════╪═══════╡
│"b"        │"11"   │
├───────────┼───────┤
│"a"        │"2"    │
└───────────┴───────┘