Question

I was trying to find a solution but did not succeed even if it seems simple. So this might be a newbie question…

I have a table userscores with 3 columns:

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

Now I need to get the difference of the points between 2012-05-01 and 2012-06-01 for each user.

Users’ points that are not existing (example userid 3) have to be calculated as 2 – 0… for this I guess I can use COALESCE(qa_points,0).

I read about combining two subqueries for the calculation but failed implementing it.

Any help appreciated.

PS: This is not working:

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;

Answer 1

I suppose your query will look like this:

    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'

LEFT JOIN is used because you told that there may be no records for this user/former date combination.