如何在Neo4j / Cypher查询中查找不同的节点

Question

I'm trying to do some pattern matching in neo4j/cypher and I came across this issue:

There are two types of graphs I want to search for:

1. Star graphs: A graph with one center node and multiple outgoing relationships.

2. n-length line graphs: A line graph with length n where none of the nodes are repeats (I have some bidirectional edges and cycles in my graph)

So the main problem is that when I do something such as:

1. MATCH a-->b, a-->c, a-->d
2. MATCH a-->b-->c-->d

Cypher doesn't guarantee (when I tried it) that a, b, c, and d are all different nodes. For small graphs, this can easily be fixed with

WHERE not(a=b) AND not(a=c) AND ...

But I'm trying to have graphs of size 10+, so checking equality between all nodes isn't a viable option. Afaik, RETURN DISTINCT does not work as well since it doesn't check equality among variables, only across different rows. Is there any simple way I can specify the query to make the differently named nodes distinct?

Answer 1

Old question, but look to APOC Path Expander procedures for how to address these kinds of use cases, as you can change the traversal uniqueness behavior for expansion (the same way you can when using the traversal API...which these procedures use).

Cypher implicitly uses RELATIONSHIP_PATH uniqueness, meaning that per path returned, a relationship must be unique, it cannot be used multiple times in a single path.

While this is good for queries where you need all possible paths, it's not a good fit for queries where you want distinct nodes or a subgraph or to prevent repeating nodes in a path.

For an n-length path, let's say depth 6 with only outgoing relationships of any type, we can change the uniqueness to NODE_PATH, where a node must be unique per path, no repeats in a path:

MATCH (n)
WHERE id(n) = 12345
CALL apoc.path.expandConfig(n, {maxLevel:6, uniqueness:'NODE_PATH'}) YIELD path
RETURN path

If you want all reachable nodes up to a certain depth (or at any depth by omitting maxLevel), you can use NODE_GLOBAL uniqueness, or instead just use apoc.path.subgraphNodes() :

MATCH (n)
WHERE id(n) = 12345
CALL apoc.path.subgraphNodes(n, {maxLevel:6}) YIELD node
RETURN node

NODE_GLOBAL uniqueness means that across all paths that a node must be unique, it will only be visited once, and there will only be one path to a node from a given start node. This keeps the number of paths that need to be evaluated down significantly, but because of this behavior not all relationships will be traversed, if they expand to a node already visited.

You will not get relationships back with this procedure (you can use apoc.path.spanningTree() for that, although as previously mentioned not all relationships will be included, as we will only capture a single path to each node, not all possible paths to nodes). If you want all nodes up to a max level and all possible relationships between those nodes, then use apoc.path.subgraphAll() :

MATCH (n)
WHERE id(n) = 12345
CALL apoc.path.subgraphAll(n, {maxLevel:6}) YIELD nodes, relationships
RETURN nodes, relationships

Richer options exist for label and relationship filtering, or filtering (whitelist, blacklist, endnode, terminator node) based on lists of pre-matched nodes.

We also support repeating sequences of relationships or node labels.

If you need filtering by node or relationship properties during expansion, then this won't be a good option as that feature is yet supported.