CODEWITHSUNDEEP
neo4jcypher
b-ryce
b-ryce

Reputation: 5828

Trying to do a MATCH inside of a nested FOREACH list

I have a set of nodes with label Type. Some of them have relationships to other Type nodes and some of them do not. I want to loop through all of the Type nodes and do some action based on the relationship (whether it is null or has an existing relationship)

In pseudo code, this is what I want to do:

For each type1 in allTypes
     For each type2 in allTypes
        Match relationship between type1 and type2   
        If relationship between type1 and type2 exists
             do something
        else
             do other thing

But I can't figure out how to do this in Cypher. I understand you can't do a MATCH inside a FOREACH statement in Cypher, so I'm trying with UNWIND like this:

MATCH (Types:Type)
WITH COLLECT(Types) AS types
UNWIND types as type1
UNWIND types as type2
MATCH (type1)-[r]->(type2)
RETURN type1.name,type(r), type2.name

This works fine for all the relationships that already exist, but when there is no relationship between type1 and type2, I'm not getting anything in my return result.

Upvotes: 0

Views: 2152

Answers (1)

Gabor Szarnyas
Gabor Szarnyas

Reputation: 5047

How about using OPTIONAL MATCH?

Some example data:

CREATE 
  (t1:Type {name:"t1"}),
  (t2:Type {name:"t2"}),
  (t3:Type {name:"t3"}),
  (t1)-[:REL]->(t2)

The query:

MATCH (t1:Type)
OPTIONAL MATCH (t1)-[r]->(t2:Type)
RETURN
  t1, t2,
  CASE r
    WHEN NULL THEN "other thing"
    ELSE "something"
  END AS info

The results:

╒══════════╤══════════╤═══════════╕
│t1        │t2        │info       │
╞══════════╪══════════╪═══════════╡
│{name: t1}│{name: t2}│something  │
├──────────┼──────────┼───────────┤
│{name: t2}│(null)    │other thing│
├──────────┼──────────┼───────────┤
│{name: t3}│(null)    │other thing│
└──────────┴──────────┴───────────┘

Upvotes: 3

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