NEO4J: Finding disconnected nodes

Question

I have this sample data

With the sample query

CREATE (a1:A {title: \"a1\"})
CREATE (a2:A {title: \"a2\"})
CREATE (a3:A {title: \"a3\"})

CRE         


        

Answer 1

Use the path pattern in where:

MATCH (a:A) WHERE NOT (a)-[:LINKS]-(:B)
RETURN a;

MATCH (b:B) WHERE NOT (b)-[:LINKS]-(:A)
RETURN b;

Or combine into one query:

OPTIONAL MATCH (a:A) WHERE NOT (a)-[:LINKS]-(:B)
WITH collect(a) AS aNodes
OPTIONAL MATCH (b:B) WHERE NOT (b)-[:LINKS]-(:A)
WITH aNodes, 
     collect(b) AS bNodes
RETURN aNodes, bNodes

Update: why the original query produces an incorrect result?

I think this is a bug. The problem is that when you use a variable for a relationship in where, the pattern implicitly uses the direction from left to right, even if it is not specified:

// Will return 0, but for test data should return 1
MATCH (b:B)-[r]-(a:A) WHERE (b)-[r]-(a)
RETURN COUNT(*);

// Will return 1
MATCH (b:B)-[r]-(a:A) WHERE (b)<-[r]-(a)
RETURN COUNT(*);

// Will return 1
MATCH (b:B)-[r]-(a:A) WHERE (b)--(a)
RETURN COUNT(*);

// Will return 1
MATCH (b:B)-[r]-(a:A) WHERE (a)-[r]-(b)
RETURN COUNT(*);