Neo4j, get all relationships between a set of nodes

Question

I've a query that fetches nodes based on a property

MATCH (c { type: 'sometype' })
WITH c LIMIT 100
RETURN c

all I want is to also fetch all the relations between nodes in the resultset, on IRC someone told me to use:

MATCH (c { type: 'sometype'])
WITH c LIMIT 100
OPTIONAL MATCH (c)-[r]-()
RETURN c, r

but that will include relationships from node c to nodes outside the resultset which in my case (some hundred thousand relationships) could create very big useless resultset or performance issues)

Is there any way to achieve that?

Answer 1

I guess there are multiple ways to do that. One approach is to find all nodes for the given type and build a collection out of them (cs). Then match again that group, this time with outgoing relationships to any node and filter that to make sure the endnode is in cs:

MATCH (c {type:'sometype'})
WITH collect(c) as cs
MATCH (x {type:'sometype'})-[r]->(y)
WHERE y in cs
RETURN startNode(r).name, endNode(r).name

Don't know your graph model, but I think it could be a good idea to refactor the property type='sometype' into a label sometype. In this case the query would look like this:

MATCH (c:Group1)   
WITH collect(c) as cs
MATCH (x:Group1)-[r]->(y)
WHERE y in cs
RETURN startNode(r).name, endNode(r).name

Answer 2

This is straight forward.

MATCH (a)-[r]-(b)
WHERE a.type = 'foo' AND b.type = 'foo'
RETURN DISTINCT r

You could equally use the new syntax:

MATCH (a { type : 'foo' }) -[r] - (b {type : 'foo'})
RETURN DISTINCT r

If you prefer it.