mysql - 使用子查询中的条件选择每组前 N 个项目

Question

我正在改编此 simpler working example 到一个稍微复杂的用例。我没有加入一个外键，而是对两个外键的成功匹配进行 IF 测试，并且在子查询中添加了 WHERE 子句，以过滤掉某些符合排名资格的行。然而，现在排名值似乎永远不会增加(因此 IF 测试总是失败)。罪魁祸首是子查询 WHERE 子句:

WHERE cand.is_pseudocandidate != 1

在下面这个查询的子查询中:

SELECT contest_id, division_id, candidate_id, n_votes, rank, n_seats 
FROM 
    (  SELECT ccd.id, contest_id, ccd.division_id, ccd.candidate_id, n_votes, cont.n_seats, 
            @rank := IF(@current_contest = ccd.contest_id AND @current_div = ccd.division_id, @rank + 1, 1) AS rank,
            @current_contest := ccd.contest_id,
            @current_div := ccd.division_id
       FROM candidates_contests_divisions ccd 
       LEFT JOIN candidates cand ON ccd.candidate_id = cand.id
       LEFT JOIN contests cont ON ccd.contest_id = cont.id
       WHERE cand.is_pseudocandidate != 1
       ORDER BY ccd.contest_id, ccd.division_id, n_votes DESC
    ) ranked  
WHERE rank <= 3;

当我注释掉该子查询 WHERE 子句时，查询按预期工作，但当然，它包含我不想要的行。

如何让这个子查询 WHERE 子句在不影响排名的情况下工作？

Here is the schema (包括测试数据)供任何人使用以使其正常工作。

Answer 1

你没有初始化排名只在left JOIN和WhERE之间放置一条线，我认为这就是全部

SELECT contest_id, division_id, candidate_id, n_votes, rank, n_seats 
FROM 
    (  SELECT ccd.id, contest_id, ccd.division_id, ccd.candidate_id, n_votes, cont.n_seats, 
            @rank := IF(@current_contest = ccd.contest_id AND @current_div = ccd.division_id, @rank + 1, 1) AS rank,
            @current_contest := ccd.contest_id,
            @current_div := ccd.division_id
       FROM candidates_contests_divisions ccd 
       LEFT JOIN candidates cand ON ccd.candidate_id = cand.id
       LEFT JOIN contests cont ON ccd.contest_id = cont.id
       CROSS JOIN ( SELECT
                      @rank := 0
                      , @current_contest := -1
                      , @current_div := -1  ) as init
       WHERE cand.is_pseudocandidate != 1
       ORDER BY ccd.contest_id, ccd.division_id, n_votes DESC
    ) ranked  
WHERE rank <= 3;