Join the DZone community and get the full member experience.
Join For FreeLearn how to build stream processing applications in Java-includes reference application. Brought to you in partnership with Hazelcast.
Neo4j 2.0 introduced the UNION (ALL) clause, which can join the results of 2 or more complete statements into a single result. Each of the statements is fully formed and contains result projection and pagination.
You need to have the same amount and names of columns to be joined in an UNION. UNION by default returns the distinct set of results.
Using UNION ALL will return the full results (and will be faster and less memory intensive).
Here are two examples:
MATCH (d:Developer) RETURN d.name as name
UNION
MATCH (e:Employee) RETURN e.name as nameMost often UNION is not needed and can be alleviated by a different graph model or query structure, but we’ll return to that later.
During a conversation with a partner of ours we found a pitfall that you might run into too. Let’s have a look at their query.
START n=node:Person(name={name})
MATCH (n)-[:MANAGES]->()-[:HAS_SKILL]->(s) RETURN s.name as name
UNION
MATCH (n)-[:HAS_SKILL]->(s) RETURN s.name as nameLooks harmless, right? The problem was it returned too much (unrelated) data.
What happened here? As we said, UNION combines the outputs from two full statements. The two statements in this case are:
// statement one
START n=node:Person(name={name})
MATCH (n)-[:MANAGES]->()-[:HAS_SKILL]->(s) RETURN s.name as name
// statement two
MATCH (n)-[:HAS_SKILL]->(s) RETURN s.name as nameAs you can see the first statement looks good, whereas the second is a full graph scan of the database, finding all HAS_SKILL relationships. Not what we want.
The direct solution would be to just fix that:
START n=node:Person(name={name})
MATCH (n)-[:MANAGES]->()-[:HAS_SKILL]->(s) RETURN s.name as name
UNION
START n=node:Person(name={name})
MATCH (n)-[:HAS_SKILL]->(s) RETURN s.name as nameBut it is much easier to just change the query structure:
START n=node:Person(name={name})
MATCH (n)-[:MANAGES*0..1]->()-[:HAS_SKILL]->(s) RETURN s.name as nameor actually:
MATCH (n:Person {name:{name}})-[:MANAGES*0..1]->
()-[:HAS_SKILL]->(s) RETURN s.name as nameWe now can even represent an arbitrary managment level structure.
MATCH (n:Person {name:{name}})-[:MANAGES*0..]->
()-[:HAS_SKILL]->(s) RETURN s.name as nameLearn how to build distributed stream processing applications in Java that elastically scale to meet demand- includes reference application. Brought to you in partnership with Hazelcast.
Like This Article? Read More From DZone
Published at DZone with permission of Michael Hunger , DZone MVB. See the original article here.
Opinions expressed by DZone contributors are their own.
{{ parent.title || parent.header.title}}
{{ parent.tldr }}
{{ parent.linkDescription }}
{{ parent.urlSource.name }}