Neo4j，Cypher：仅匹配包含特定“用户”的“组”

Question

I have (:User) nodes which are grouped [:IN] (:Group) nodes.

CREATE (u1:User {name: 'u1'}), (u2:User {name: 'u2'}), (u3:User {name: 'u3'})
CREATE (g1:Group {name: 'g1'}), (g2:Group {name: 'g2'})
CREATE u1-[:IN]->g1, u2-[:IN]->g1, u3-[:IN]->g1
CREATE u1-[:IN]->g2, u2-[:IN]->g2
RETURN *

Given "u1" and "u2" I want to find out which (:Group) they ALONE belong to, ie "g2." My first attempt is INCORRECT :

MATCH (u:User)
WHERE u.name IN ['u1', 'u2']
WITH u
MATCH (u)-[:IN]->(g:Group)
RETURN g, collect(u)

It understandably returns the two (:Group) 's these users belong to.

How would find out that "u1" and "u2" alone belong to "g2?"

Answer 1

You can try a criterion like "both 'u1' and 'u2' are members of the group and there are only two members of the group". Here's one way to say that in cypher

MATCH (:User { name:'u1' })-[:IN]->(g)<-[:IN]-({ name:'u2' })
MATCH g<-[:IN]-(m)
WITH g, collect(m) AS members
WHERE length(members)= 2
RETURN *

Answer 2

MATCH (g:Group)<--(m:User)
WITH g, collect(m) AS m
WHERE length(m) = 2 AND ALL (u IN m WHERE u.name IN ['u1','u2'])
RETURN *

to make sure that the group not only contains 2 users but those users are u1 and u2