Neo4j和Cypher - 可选匹配的奇怪行为

Question

Today I was writing DELETE query for some parts of my graph but I am having some problems with OPTIONAL MATCH .

Firstly here are example graphs:

http://console.neo4j.org/r/micnvv

and

http://console.neo4j.org/?id=og6d9s

I wanted to write a query that deletes all foo and bar instances, problem is that sometimes fooParent will not exists and also there are cases when single foo will not be connected to any bar . Because of those conditions I decided to match fooParent and bar nodes in OPTIONAL MATCH query.

Now in first graph (where fooParent and bar nodes are present) everything I want is matched ( foo1 and all bar nodes) using

MATCH (foo:Foo { customId: '1' })
OPTIONAL MATCH foo -[rel]-> bar,(fooParent: FooParent)-[fooParentRel]-> foo
RETURN foo, bar

In the second graph I have a situation where fooParent for given foo does not exists and the same query does not match bar nodes - only foo is matched as you can see.

I was thinking that OPTIONAL MATCH is the way to go in case such as mine but it does not seem to be working.

Answer 1

Well yes, because an OPTIONAL MATCH clause either

• completely matches, in which case it will return the matching rows with every identifier bound, or
• it will not completely match, in which case it will return a single row with any unbound identifiers set to null .

In this case, your OPTIONAL MATCH contains two parts: it tries to match Foo nodes that have outgoing relationships (to some bar) and incoming relationships to a FooParent .

The solution is to split up the OPTIONAL MATCH :

MATCH (foo:Foo { customId: '1' })
OPTIONAL MATCH foo -[rel]-> bar
OPTIONAL MATCH (fooParent: FooParent)-[fooParentRel]-> foo
RETURN foo, bar

Answer 2

In the second graph, by running two distinct optional match functions the following query will work

MATCH (foo:Foo { customId: '1' })

OPTIONAL

MATCH (foo)-[r]->(bar)

OPTIONAL

MATCH (fooParent:fooParent)-[fooParentRel]->(foo2)

RETURN foo, bar, foo2

Cheers,

Chris