Match pairs of nodes with reciprocal relationships

Question

I have a data model where nodes of the same label are connected by relationships of the same type:

There is no hierarchy or 'logic' which of the :Concept nodes is the start node of the reciprocal relationship.

My goal is to get all unique pairs of nodes. Wen you match on the :Concept nodes, you get two pairs, one starting at 123 the other starting at 234 :

MATCH (c1:Concept)-[:REL]->(c2:Concept)
RETURN [c1.concept_id, c2.concept_id]

Result:

[123, 234]
[234, 123]

Is there a way to get each pair only once? The order of the pairs does not matter.

Answer 1

You could do this:

MATCH (c1:Concept)-[:REL]-(c2:Concept)
WHERE id(c1) > id(c2)
RETURN [c1.concept_id, c2.concept_id]