Cypher Count with distinct and nulls

[kantube]

Cypher 1.6

i want to count distinct nodes but ran in to an issue if the match clause includes optional relationships, for example:

start a = node(0)
match a-[:friend]->b-[?:like]->d
return count(distinct b), count(distinct d)

if there is a friend that has no "like" this throws an error "Flat hash tables cannot contain null elements"

this is just a sample query but demonstrates my issue.  i am trying to do some reporting and summing up of the data and need to count distinct elements.  if i remove the distinct my counts are incorrect (well correct if you want to count duplicates).

is there a way to do this with cypher?  if not can i do this with gremlin?

thanks

[Marko Rodriguez]

Hi,

Here is the respective Gremlin form:

g.V.out('knows').as('a').out('created').as('b').optional(1).table(t)

Here it is in action over the toy TinkerPop graph:

https://github.com/tinkerpop/gremlin/wiki/Defining-a-Property-Graph

gremlin> g = TinkerGraphFactory.createTinkerGraph()

==>tinkergraph[vertices:6 edges:6]

gremlin> t = new Table()

gremlin> g.V.out('knows').as('a').out('created').as('b').optional(1).table(t).iterate()

==>null

gremlin> t 

==>[a:v[2], b:null]

==>[a:v[4], b:v[3]]

gremlin> t.getColumn(0)._().dedup.count()

==>2

gremlin> t.getColumn(1)._().dedup.count()

==>2

HTH,

Marko.

http://markorodriguez.com

[Andres Taylor]

On Tue, Feb 7, 2012 at 6:23 PM, kantube <mic...@mkanner.com> wrote:

Cypher 1.6

i want to count distinct nodes but ran in to an issue if the match clause includes optional relationships, for example:

start a = node(0)
match a-[:friend]->b-[?:like]->d
return count(distinct b), count(distinct d)

That should definitely work. Sorry you had to run into a bug.

https://github.com/neo4j/community/issues/214

Andrés

 

[kantube]

will this fix be in a snapshot that i can get?  thx.


the error message that was thrown by this error was a lot better then the error message of not saving puppies like this one gives

• start a = node(0)
• match a-[:friend]->b

• where b-[:like]->() and ().Name? = 'myName'
• return a, b

this was just some fat fingers on copy/paste not intending to harm puppies