Question

I have a query that is pulling users who liked a specific object from a users table. Ratings are stored in a ratings table. The query I have come up with so far looks like this:

SELECT user.id, user.name, user.image
  FROM users
LEFT JOIN ratings ON ratings.userid = user.id
WHERE rating.rating > 0
  AND rating.objectId IN (1,2,3,4)

I want to be able to put a LIMIT on this query, to avoid returning all the results, when I only need 3 or so results for each ID. If I just put a LIMIT 12 for example, I might get 8 records with one id, and 1 or 2 each for the others – i.e. an uneven distribution across the IDs.

Is there a way to write this query so as to guarantee that (assuming an object has been “liked” at least three times), I get three results for each of the ids in the list?

Answer 1

By setting the row number whit variables, and then filter that result to show only row 1-3 should work

SET @last_objectId = 0;
SET @count_objectId = 0;
SELECT id, name, image FROM (
SELECT
 user.id,
 user.name,
 user.image,
 @count_objectId := IF(@last_objectId = rating.objectId, @count_objectId, 0) + 1 AS rating_row_number,
 @last_objectId := rating.objectId
FROM users
LEFT JOIN ratings ON (ratings.userid = user.id)
WHERE
 rating.rating > 0 AND
 rating.objectId IN (1,2,3,4)
ORDER BY rating.objectId
) AS subquery WHERE rating_row_number <= 3;