Limit the results of a union cypher query

Question

Let's say we have the example query from the documentation:

MATCH (n:Actor)
RETURN n.name AS name
UNION
MATCH (n:Movie)
RETURN n.title AS name

I know that if I do that:

MATCH (n:Actor)
RETURN n.name AS name
LIMIT 5
UNION
MATCH (n:Movie)
RETURN n.title AS name
LIMIT 5

I can reduce the returned results of each sub query to 5.How can I LIMIT the total results of the union query?

Answer 1

This is not yet possible, but there is already an open neo4j issue that requests the ability to do post-UNION processing, which includes what you are asking about. You can add a comment to that neo4j issue if you support having it resolved.

Answer 2

This can be done using UNION post processing by rewriting the query using the COLLECT function and the UNWIND clause.

First we turn the columns of a result into a map (struct, hash, dictionary), to retain its structure. For each partial query we use the COLLECT to aggregate these maps into a list, which also reduces our row count (cardinality) to one (1) for the following MATCH. Combining the lists is a simple list concatenation with the “+” operator.

Once we have the complete list, we use UNWIND to transform it back into rows of maps. After this, we use the WITH clause to deconstruct the maps into columns again and perform operations like sorting, pagination, filtering or any other aggregation or operation.

The rewritten query will be as below:

MATCH (n:Actor)
with collect ({name: n.title}) as row

MATCH (n:Movie)
with row + collect({name: n.title}) as rows

unwind rows as row
with row.name as name
return name LIMIT 5

Answer 3

This is possible in 4.0.0

CALL {
  MATCH (p:Person) RETURN p 
  UNION
  MATCH (p:Person) RETURN p 
}
RETURN p.name, p.age ORDER BY p.name

Read more about Post-union processing here https://neo4j.com/docs/cypher-manual/4.0/clauses/call-subquery/