Neo4j: Cypher query returns duplicate results

Question

I have two graphs built like this :

CREATE (level1a:Bug {name: 'a'})
CREATE (level1b:Bug {name: 'b'})
CREATE (level2c:Bug {name: 'c'})
CREATE (level2d:Bug {name: 'd'})
CREATE (level3e:Bug {name: 'e'})
CREATE (level3f:Bug {name: 'f'})
CREATE (level3g:Bug {name: 'g'})
CREATE (level3h:Bug {name: 'h'})

CREATE (level1a)-[:LINK]->(level2c)
CREATE (level1b)-[:LINK]->(level2d)
CREATE (level2c)-[:LINK]->(level3e)
CREATE (level2c)-[:LINK]->(level3f)
CREATE (level2d)-[:LINK]->(level3g)
CREATE (level2d)-[:LINK]->(level3h)

And also available here : http://console.neo4j.org/?id=duplicate_bug2

When I execute the query :

MATCH (a:Bug {name: 'a'})-[:LINK]->()-[:LINK]->(end) return end

I get the expected two nodes (f and e). But if I do two match queries like this :

MATCH (a:Bug {name: 'a'})-[:LINK]->()-[:LINK]->(end)
MATCH (b:Bug {name: 'b'})-[:LINK]->()-[:LINK]->(end2)
return end, end2

I get duplicates nodes in end and end2. Why is this? The two graphs are not even connected!

BR, S

Dave Bennett Dave Bennett · Accepted Answer · 2016-07-01T12:22:14

Since both matches will return multiple rows and there is no correlation between the two match statements it will generate a cross product of the two result sets. In this case it is 2x2 so you get four rows of each node with each node.

I think what you are after is something like this query. It finds all of the ends from the first match, combines them in a collection and then repeats the process for the second match. Then it returns a single row in the result set with all of the ends of a and all of the ends of b regardless of how many there are at the end of each match.

MATCH (a:Bug {name: 'a'})-[:LINK]->()-[:LINK]->(end)
with collect(end) as end
MATCH (b:Bug {name: 'b'})-[:LINK]->()-[:LINK]->(end2)
return  end, collect(end2) as end2

Neo4j: Cypher query returns duplicate results

1 Answers