从每组 sqlite 中选择前 n 条记录

Question

我正在尝试从如下所示的数据库表结果中选择前 2 条记录

SubjectId |  StudentId | Levelid | total
------------------------------------------
 1        |  1         |   1     | 89
 1        |  2         |   1     | 77
 1        |  3         |   1     | 61
 2        |  4         |   1     | 60
 2        |  5         |   1     | 55
 2        |  6         |   1     | 45

我试过这个查询

SELECT rv.subjectid,
       rv.total,
       rv.Studentid,
       rv.levelid
  FROM ResultView rv
       LEFT JOIN ResultView rv2
              ON ( rv.subjectid = rv2.subjectid 
    AND
rv.total <= rv2.total ) 
 GROUP BY rv.subjectid,
          rv.total,
          rv.Studentid
HAVING COUNT( * ) <= 2
order by rv.subjectid desc  

但有些主题，例如缺少的地方，我什至尝试了以下链接的建议

How to select the first N rows of each group?

但我得到的每个 subjectid 多于两个

我做错了什么？

Answer 1

希望我能正确理解您的问题。让我知道这是否正确：

我重新创建了你的表：

CREATE TABLE stack (
       SubjectId INTEGER(10),
       StudentId INTEGER(10),
       Levelid INTEGER(10),
       total INTEGER(10)
       )
;

插入的值

INSERT INTO stack VALUES
       (1,1,1,89),
       (1,2,1,77),
       (1,3,1,61),
       (2,4,1,60),
       (2,5,1,55),
       (2,6,1,45)
;

如果您尝试按 Levelid 获取顶级组（按总字段排序，假设 StudentID 作为主键）：

SELECT * 
FROM stack AS a
WHERE a.StudentID IN (
      SELECT b.StudentID
      FROM stack AS b
      WHERE a.levelid = b.levelid
      ORDER BY b.total DESC
      LIMIT 2
      )
;

产生这个结果：

SubjectId | StudentId | Levelid | total
1         | 1         | 1       | 89
1         | 2         | 1       | 77

SubjectId 前 2 名的示例，按总数排序：

SELECT * 
FROM stack AS a
WHERE a.StudentID IN (
      SELECT b.StudentID
      FROM stack AS b
      WHERE a.subjectID = b.subjectID
      ORDER BY b.total DESC
      LIMIT 2
      )
;

结果：

SubjectId | StudentId | Levelid | total
1         | 1         | 1       | 89
1         | 2         | 1       | 77
2         | 4         | 1       | 60
2         | 5         | 1       | 55

我希望这就是您正在寻找的答案。

Answer 2

您可以使用相关子查询：

select  *
from    ResultView rv1
where   SubjectId || '-' || StudentId || '-' || LevelId in
        (
        select  SubjectId || '-' || StudentId || '-' || LevelId
        from    ResultView rv2
        where   SubjectID = rv1.SubjectID
        order by
                total desc
        limit   2
        )

此查询通过连接三列来构造单列主键。如果你有一个真正的主键（比如ResultViewID），你可以用它代替SubjectId || '-' || StudentId || '-' || LevelId。

Example at SQL Fiddle.