Get common lines in the same table

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I've had a bit of a search, but didn't find anything quite like what I'm trying to achieve.

Basically, I'm trying to find a similarity between two users' voting habits.

I have a table storing each individual vote made, which stores:

voteID
itemID     (the item the vote is attached to)
userID     (the user who voted)
direction  (whether the user voted the post up, or down)

I'm aiming to calculate the similarity between, say, users A and B, by finding out two things:

  1. The number of votes they have in common. That is, the number of times they've both voted on the same post (the direction does not matter at this point).
  2. The number of times they've voted in the same direction, on common votes.

(Then simply to calculate #2 as a percentage of #1, to achieve a crude similarity rating).

My question is, how do I find the intersection between the two users' sets of votes? (i.e. how do I calculate point #1 adequately, without looping over every vote in a highly inefficient way.) If they were in different tables, an INNER JOIN would suffice, I'd imagine... but that obviously won't work on the same table (or will it?).

Any ideas would be greatly appreciated.


Something like this:

SELECT COUNT(*)
FROM votes v1
INNER JOIN votes v2 ON (v1.item_id = v2.item_id)
WHERE v1.userID = 'userA'
AND v2.userUD = 'userB'