MySQL: Last 10 entries per user?

Question

I have a table storing transactional data for users. To find a users rating you take the average score of the last 10 entries for that user. Is there a way to get that with SQL?

I need to be able to work it out for a single user given their ID. And to get a list of all users ordered by their score.

Currently I'm working it out outside MySQL and storing it in another col for each user so I can ORDER_BY that.

# returns average for all transactions.
SELECT user_id, AVG(score) FROM transactions WHERE user_id = 1
# returns scores for last 10 transactions.
SELECT user_id, score FROM transactions WHERE user_id = 1 ORDER_BY date DESC LIMIT 10

Answer 1

Just combine the two queries already in use:

SELECT user_id, AVG(score)
FROM transactions
WHERE rowid in (
    SELECT rowid
    FROM transactions
    WHERE user_id = 1
    ORDER_BY date DESC
    LIMIT 10)

where rowid is whatever the identifying I.D. is.

Answer 2

Use:

  SELECT x.user_id,
         AVG(x.score)
    FROM (SELECT t.user_id,
                 t.score
            FROM TRANSACTIONS t
           WHERE t.user_id = ?
        ORDER BY t.date
           LIMIT 10) x
GROUP BY x.user_id