Check if two nodes are directly connected in cypher

[mixalisnikolaou]

Hi everyone,

I'm currently starting to learn how Cypher works and I got stuck trying to find if 2 specific nodes  are connected directly.  I want to get 1 if the nodes are connected and 0 if they are not.

I write the following but this just find all the neigbors . 

MATCH (n where ID(n)=1000)
CALL apoc.path.subgraphNodes(n, {maxLevel: 1}) YIELD node
RETURN node 

Could anyone help me find a solution how to do this?

Thank you very much.

[glilienfield]

If you want to find out if two specific nodes are connected directly (one hop), then you need to find both nodes and test for the existence of a relationships between them.  The following query does that. It uses the 'exists' method to determine if a direct connection exists between nodes 'm' and 'n'.  The result is either 'true' or 'false'. 

match (n where ID(n)=1000)
match (m where ID(m)=1001)
return exists( (n)--(m) ) 

If you really want '0' or '1' instead, you can use a 'case' statement to map true/false to 1/0. 

match (n where ID(n)=1000)
match (m where ID(m)=1001)
return case exists( (n)--(m) ) when true then 1 when false then 0 end

Or, a slightly different syntax:

match (n where ID(n)=1000)
match (m where ID(m)=1001)
with exists( (n)--(m) ) as isConnected
return case isConnected when true then 1 when false then 0 end

BTW, (n)--(m) is shorthand notation of (n)-[]-(m), which represents a direct connection between 'n' and 'm'. 

[mixalisnikolaou]

Thank you this works perfect for me

[Cobra]

Hello @mixalisnikolaou 😊

If the nodes are densely connected, you should use apoc.nodes.connected() function from the APOC plugin:

MATCH (n WHERE id(n) = 1000) 
MATCH (m WHERE id(m) = 1001) 
RETURN apoc.nodes.connected(n, m) AS output;

Regards,
Cobra