Question

我正在将此 simpler working example 改编为稍微复杂的用例。我没有加入一个外键，而是在成功匹配两个外键的情况下进行IF测试，并且我在子查询的WHERE子句中添加了过滤掉某些行以符合排名的条件。但是，现在等级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

在此查询的子查询中：

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;

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

如何在不干扰排名的情况下使此子查询WHERE子句正常工作？

Here is the schema（包括测试数据）供所有人使用以实现此功能。

Answer 1

你没有初始化排名只在左边JOIN 和面对面之间放置一条线我认为那就是

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;

使用子查询中的条件为每个组选择前N个项目

1 个答案: