在有向图中找到树的根

Question

我有一个像节点（1） - >节点（2） - >节点（3）的树结构。我有一个名称作为用于检索节点的属性。给定节点say节点（3），我想检索节点（1）。

查询尝试：

MATCH（P：节点） - [：HAS *] - >（C：节点）WHERE c.name = “节点3” RETURN p LIMIT 5

但是，不能够获得节点1.

Answer 1

您的查询不仅会返回“节点1”，但它至少应包含一个包含它的路径。这是可能的筛选路径只得到了一个一路穿越到根，但是：

MATCH (c:Node {name: "node 3"})<-[:HAS*0..]-(p:Node) 
// The root does not have any incoming relationship 
WHERE NOT (p)<-[:HAS]-() 
RETURN p 

注意使用0长度的，它匹配所有的情况下，包括一个在起始节点是的根。有趣的事实：即使你在Node：name上有一个索引，它也不会被使用（除非你使用的是Neo4j 3.1，它至少在3.1 Beta2之后似乎是固定的），你必须明确地指定它。

MATCH (c:Node {name: "node 3"})<-[:HAS*0..]-(p:Node) 
USING INDEX c:Node(name) 
WHERE NOT (p)<-[:HAS]-() 
RETURN p 

第一查询使用PROFILE（带有一个数字id属性，而不是name）：

+-----------------------+----------------+------+---------+-------------------------+----------------------+ 
| Operator    | Estimated Rows | Rows | DB Hits | Variables    | Other    | 
+-----------------------+----------------+------+---------+-------------------------+----------------------+ 
| +ProduceResults  |    0 | 1 |  0 | p      | p     | 
| |      +----------------+------+---------+-------------------------+----------------------+ 
| +AntiSemiApply  |    0 | 1 |  0 | anon[23], c -- p  |      | 
| |\     +----------------+------+---------+-------------------------+----------------------+ 
| | +Expand(All)  |    1 | 0 |  3 | anon[58], anon[67] -- p | (p)<-[:HAS]-()  | 
| | |     +----------------+------+---------+-------------------------+----------------------+ 
| | +Argument   |    1 | 3 |  0 | p      |      | 
| |      +----------------+------+---------+-------------------------+----------------------+ 
| +Filter    |    1 | 3 |  3 | anon[23], c, p   | p:Node    | 
| |      +----------------+------+---------+-------------------------+----------------------+ 
| +VarLengthExpand(All) |    1 | 3 |  5 | anon[23], p -- c  | (c)<-[:HAS*]-(p)  | 
| |      +----------------+------+---------+-------------------------+----------------------+ 
| +Filter    |    1 | 1 |  3 | c      | c.id == { AUTOINT0} | 
| |      +----------------+------+---------+-------------------------+----------------------+ 
| +NodeByLabelScan  |    3 | 3 |  4 | c      | :Node    | 
+-----------------------+----------------+------+---------+-------------------------+----------------------+ 

Total database accesses: 18 

，并在第二个：

+-----------------------+----------------+------+---------+-------------------------+------------------+ 
| Operator    | Estimated Rows | Rows | DB Hits | Variables    | Other   | 
+-----------------------+----------------+------+---------+-------------------------+------------------+ 
| +ProduceResults  |    0 | 1 |  0 | p      | p    | 
| |      +----------------+------+---------+-------------------------+------------------+ 
| +AntiSemiApply  |    0 | 1 |  0 | anon[23], c -- p  |     | 
| |\     +----------------+------+---------+-------------------------+------------------+ 
| | +Expand(All)  |    1 | 0 |  3 | anon[81], anon[90] -- p | (p)<-[:HAS]-() | 
| | |     +----------------+------+---------+-------------------------+------------------+ 
| | +Argument   |    1 | 3 |  0 | p      |     | 
| |      +----------------+------+---------+-------------------------+------------------+ 
| +Filter    |    1 | 3 |  3 | anon[23], c, p   | p:Node   | 
| |      +----------------+------+---------+-------------------------+------------------+ 
| +VarLengthExpand(All) |    1 | 3 |  5 | anon[23], p -- c  | (c)<-[:HAS*]-(p) | 
| |      +----------------+------+---------+-------------------------+------------------+ 
| +NodeUniqueIndexSeek |    1 | 1 |  2 | c      | :Node(id)  | 
+-----------------------+----------------+------+---------+-------------------------+------------------+ 

Total database accesses: 13