Question

I have a table called ratings that looks like this

Rating   |   Picid    |   User
   8           12          6
   7           12          6
   9           15          7
   5           16          7
   9           17          8
   10          2           8
   7           18          3
   10          22          12

I want to group the ratings based on the picid, but only choose the highest rated picid per user. I have the query like this already:

SELECT *, AVG(rating) AS total FROM ratings
  GROUP BY picid ORDER by total DESC

Right now the query would output the information like this:

Rating   |   Picid    |   User
 7.5           12          6
   9           15          7
   5           16          7
   9           17          8
   10          2           8
   7           18          3
   10          22          12

So it successfully groups the ratings based on the picid, but I want only the ONE highest per user. I want it to output information like this:

Rating   |   Picid    |   User
 7.5           12          6
   9           15          7
   10          2           8
   7           18          3
   10          22          12

See how it only allows the highest rated picid per user?
How can I modify my query to do this?

Let me know if you need more clarification on what I’m asking.

Answer 1

You first must calculate the average ratings for each (Picid,User) combination, then find the group-wise maximum over that materialised result:

SELECT Rating, PicID, User
FROM (
  SELECT   AVG(Rating) AS Rating, Picid, User
  FROM     ratings
  GROUP BY Picid, User
) t1 NATURAL JOIN (
  SELECT   MAX(Rating) AS Rating, User
  FROM     (
    SELECT   AVG(Rating) AS Rating, User
    FROM     ratings
    GROUP BY Picid, User
  ) t
  GROUP BY User
) t2

See it on sqlfiddle.