CODEWITHSUNDEEP
neo4jcypher
Marek Grzenkowicz
Marek Grzenkowicz

Reputation: 17353

Why is a single Neo4j relationship shown twice in Cypher query results?

Let's consider a trivial graph with a directed relationship:

CREATE 
  (`0` :Car {value:"Ford"})
, (`1` :Car {value:"Subaru"})
, (`0`)-[:`DOCUMENT` {value:"DOC-1"}]->(`1`);

The following query MATCH (n1:Car)-[r:DOCUMENT]-(n2:Car) RETURN * returns:

╒══════════════════╤══════════════════╤═════════════════╕
│"n1"              │"n2"              │"r"              │
╞══════════════════╪══════════════════╪═════════════════╡
│{"value":"Subaru"}│{"value":"Ford"}  │{"value":"DOC-1"}│
├──────────────────┼──────────────────┼─────────────────┤
│{"value":"Ford"}  │{"value":"Subaru"}│{"value":"DOC-1"}│
└──────────────────┴──────────────────┴─────────────────┘

The graph defines only a Ford->Subaru relationship, why are two relationships?
How to interpret the reversed one (line 1; not specified in the CREATE) statement?

Note: This is a follow-up to Convert multiple relationships between 2 nodes to a single one with weight asked by me earlier. I solved my problem, but I'm not convinced my answer is the best solution.

Upvotes: 0

Views: 717

Answers (1)

InverseFalcon
InverseFalcon

Reputation: 30397

Your MATCH statement here doesn't specify the direction, therefore there are two possible paths that will match the pattern (remember that the ordering of nodes in the path is important and distinguishes paths from each other), thus your two answers.

If you specify the direction of the relationship instead you'll find there is only one possible path that matches:

MATCH (n1:Car)-[r:DOCUMENT]->(n2:Car) 
RETURN *

As for the question of why we get two paths back when we omit the direction, remember that paths are order-sensitive: two paths that have the same elements but with a different order of the elements are different paths.

To help put this into perspective, consider the following two queries:

# Query 1
MATCH (n1:Car)-[r:DOCUMENT]-(n2:Car) 
WHERE n1.value = 'Ford'
RETURN *

╒══════════════════╤══════════════════╤═════════════════╕
│"n1"              │"n2"              │"r"              │
╞══════════════════╪══════════════════╪═════════════════╡
│{"value":"Ford"}  │{"value":"Subaru"}│{"value":"DOC-1"}│
└──────────────────┴──────────────────┴─────────────────┘

# Query 2
MATCH (n1:Car)-[r:DOCUMENT]-(n2:Car) 
WHERE n1.value = 'Subaru'
RETURN *

╒══════════════════╤══════════════════╤═════════════════╕
│"n1"              │"n2"              │"r"              │
╞══════════════════╪══════════════════╪═════════════════╡
│{"value":"Subaru"}│{"value":"Ford"}  │{"value":"DOC-1"}│
└──────────────────┴──────────────────┴─────────────────┘

Conceptually (and also used by the planner, in absence of indexes), to get to each of the above results you start off with the results of the full match as in your description, then filter to the only one which meets the given criteria.

The results above would not be consistent with the original directionless match query if that original query only returned a single row instead of two.

Additional information from the OP

It will take a while to wrap my head around it, but it does work this way and here's a piece of documentation to confirm it's by design:

When your pattern contains a bound relationship, and that relationship pattern doesn’t specify direction, Cypher will try to match the relationship in both directions.

MATCH (a)-[r]-(b)
WHERE id(r)= 0
RETURN a,b

This returns the two connected nodes, once as the start node, and once as the end node.

Upvotes: 1

Related Questions