Active Record 查询 Distinct Group by

Question

I have a query, where I am trying to find min score of a user in a grade, in a grade, there are users with the same min score

Example: User A has a score of 2 and user B has a score of 2, so my expectation is to get both the users grouped by grade.

However, I am only getting one user. The query is:

users = Users.all
@user_score = users
      .where.not(score: [ nil, 0 ])
      .select('DISTINCT ON ("users"."grade") grade, "users".*')
      .order('"users"."grade" ASC, "users"."score" ASC')
      .group_by(&:grade)

Please if some can guide me what am i doing wrong here.

Answer 1

DISTINCT will cut off all non uniq values in the result, so there is no way to get multiple users with same min score in your query.

I think you can achieve the desired result with window function:

SELECT * FROM
 (SELECT *, rank() OVER (PARTITION BY grade ORDER BY score) AS places 
  FROM users
  WHERE score IS NOT NULL AND score != 0
 ) AS ranked_by_score
WHERE places = 1;