in reply to sql adjacency list

Aristotle's answer is bang-on, for a SQL solution. There are other ways.

If

you might want to consider loading everything into a Graph::Undirected and using the graph methods yourself. I'd look at the set of the vertices a distance of 2 from a given vertex. It's really easy to write this method and it will run in near-constant (and un-noticeable) time.

The major improvement that this has over a SQL solution is that it will scale better the more friendships you have. If you have, say, 50 people in your database and each is friends with 20 people, you have 50 vertices and 1000 edges. If each person meets 5 people, you still have 50 people, but 1250 edges. If done poorly, the friendship table in Aristotle's solution will bog you down.

I also have a version of Graph::Undirected* that is optimized for this type of analysis (unique edges between vertices) which is up to 4x faster than the generic Graph::Undirected on CPAN.

*: Jarrko's aware of it.

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