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MySQL: Get difference between two values in one table (multiple userids)?
date          userid   points
2012-05-01    1        23
2012-06-01    1        34
2012-07-01    1        44
2012-05-01    2        78
2012-06-01    2        94
2012-07-01    2        99
2012-06-01    3         2
2012-07-01    3         9

SELECT t1.userid, t1.points - t2.points AS mpoints FROM (
        SELECT userid,points FROM `userscores`
        WHERE YEAR(date) = YEAR('2012-05-01')
        AND MONTH(date) = MONTH('2012-05-01') )
        AS t1
    JOIN (
        SELECT userid,points FROM `userscores`
        WHERE YEAR(date) = YEAR('2012-04-01')
        AND MONTH(date) = MONTH('2012-04-01') )
        AS t2
    ORDER BY mpoints DESC, t1.userid DESC;

SELECT ul.userid,
           ul.points - COALESCE(uf.points, 0) AS points_difference
      FROM userscores ul
 LEFT JOIN
           (SELECT userid, points FROM userscores WHERE `date` = '2012-05-01') AS uf
        ON uf.userid = ul.userid
     WHERE ul.date = '2012-06-01'