Neo4j Cypher: Group nodes by the relation to another node

Question

Having a graph like post-->category , how can I get one post per category?

ie:

Having
    Post A1 --> Category A
    Post A2 --> Category A
    Post B1 --> Category B
    Post B2 --> Category B
    Post B3 --> Category B
    Post C1 --> Category C

I should get Post A2, Post B1, Post C1. 

I don't mind what post I get for a given category, just to get one for each category.

Thanks!

Answer 1

To pick a post per category by random:

MATCH (p:Post)-[:HAS_CATEGORY]->(c:Category)
WITH c, collect(p) as posts
RETURN c, posts[toInt(rand()*length(posts))]

We're using the collect aggregation function per category and in the return we pick one entry by random.