Random Thoughts (Ordering) in Cypher
Random Thoughts (Ordering) in Cypher
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There was a question on the Neo4j Google Group about returning results in a random order in Cypher. So I thought explaining it in a blog post (this) and an interactive GraphGist is better than just to answer the email.
The naive approach is just to order by rand():
MATCH (n:Crew)
RETURN n
ORDER BY rand()Which fails with:
ORDER BY expressions must be deterministic. For instance, you cannot use the rand() function in the expressionThat’s explainable, how would you order by something that generates a new value on each call. So we have to associate a random value with each row. Fortunately that’s easy with WITH:
MATCH (n:Crew)
WITH n, rand() AS r
ORDER BY r
RETURN nSo WITH n, rand() as r adds a new column called r (we need the alias) to each row, then we can order by that as part of with (we could even paginate) but in the RETURN clause we only return n and don’t expose r.
This approach is also handy if you want to order by things that you don’t want to expose later on.
Like ordering by the name of the person, but returning the friends in that order:
MATCH (n:Crew)-[:KNOWS]->(o)
WITH n.name as name, o
ORDER BY name DESC
RETURN oDownload Building Reactive Microservices in Java: Asynchronous and Event-Based Application Design. Brought to you in partnership with Red Hat.
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