Limiting a Neo4j cypher query results by sum of relationship property

Question

Is there a way to limit a cypher query by the sum of a relationship property?

I'm trying to create a cypher query that returns nodes that are within a distance of 100 of the start node. All the relationships have a distance set, the sum of all the distances in a path is the total distance from the start node.

If the WHERE clause could handle aggregate functions what I'm looking for might look like this

START n=node(1)
MATCH path = n-[rel:street*]-x
WHERE SUM( rel.distance ) < 100
RETURN x

Is there a way that I can sum the distances of the relationships in the path for the where clause?

Answer 1

Sure, what you want to do is like a having in a SQL query.

In cypher you can chain query segments and use the results of previous parts in the next part by using WITH , see the manual .

For your example one would assume:

START n=node(1)
MATCH n-[rel:street*]-x
WITH SUM(rel.distance) as distance
WHERE distance < 100
RETURN x

Unfortunately sum doesn't work with collections yet

So I tried to do it differently (for variable length paths):

START n=node(1)
MATCH n-[rel:street*]-x
WITH collect(rel.distance) as distances
WITH head(distances) + head(tail(distances)) + head(tail(tail(distances))) as distance
WHERE distance < 100
RETURN x

Unfortunately head of an empty list doesn't return null which could be coalesce d to 0 but just fails. So this approach would only work for fixed length paths, don't know if that's working for you.

Answer 2

I've come across the same problem recently. In more recent versions of neo4j this was solved by the extract and reduce clauses. You could write:

START n=node(1)
MATCH path = (n)-[rel:street*..100]-(x)
WITH extract(x in rel | x.distance) as distances, x
WITH reduce(res = 0, x in rs | res + x) as distance, x
WHERE distance <100
RETURN x

Answer 3

i dont know about a limitation in the WHERE clause, but you can simply specify it in the MATCH clause:

START n=node(1)
MATCH path = n-[rel:street*..100]-x
RETURN x

see http://docs.neo4j.org/chunked/milestone/query-match.html#match-variable-length-relationships