如何选择每组的顶部和底部记录？

Question

说，我在数据库（SQL Server 2008）中有一个表，其中的数据与此类似 （但要大得多）：

| ID | SCORE | GROUP |
-----------------------
| 10 |     1 | A     |
| 6  |     2 | A     |
| 3  |     3 | A     |
|----|-------|-------|
| 8  |     5 | B     |
|----|-------|-------|
| 4  |     1 | C     |
| 9  |     3 | C     |
| 2  |     4 | C     |
| 7  |     4 | C     |
|----|-------|-------|
| 12 |     3 | D     |
| 1  |     3 | D     |
| 11 |     4 | D     |
| 5  |     6 | D     |

我想获取每个GROUP 的顶部和底部记录的ID，其中每个组的记录按SCORE 排序（以及补充，ID），如下所示：

| GROUP | MIN_ID | MAX_ID  |
----------------------------
| A     | 10     | 3       |
| B     | 8      | 8       |
| C     | 4      | 7       |
| D     | 1      | 5       |

问题是：我怎样才能做到这一点？

到目前为止，我一直在尝试基于 RANK() 函数的解决方案，但还没有找到一个既能产生正确输出又具有模糊效率或可维护性的查询。

---

注意事项：

这个例子被简化了。我的“表”实际上是一个已经很复杂的查询的输出，我希望将最后阶段添加到该查询中。我宁愿只从表格中选择一次。

如果可能，最好有一个通用的解决方案，让我可以选择每个组的顶部和底部 n 值。

IDs 的顺序不方便。

Answer 1

您可以使用子查询 (SELECT TOP 1)...

Answer 2

DECLARE @YourTable TABLE (ID INTEGER, Score INTEGER, [Group] VARCHAR(1))
INSERT INTO @YourTable VALUES (10, 1, 'A')
INSERT INTO @YourTable VALUES (6 , 2, 'A')
INSERT INTO @YourTable VALUES (3 , 3, 'A')
INSERT INTO @YourTable VALUES (8 , 5, 'B')
INSERT INTO @YourTable VALUES (4 , 1, 'C')
INSERT INTO @YourTable VALUES (9 , 3, 'C')
INSERT INTO @YourTable VALUES (2 , 4, 'C')
INSERT INTO @YourTable VALUES (7 , 4, 'C')
INSERT INTO @YourTable VALUES (12, 3, 'D')
INSERT INTO @YourTable VALUES (1 , 3, 'D')
INSERT INTO @YourTable VALUES (11, 4, 'D')
INSERT INTO @YourTable VALUES (5 , 6, 'D')  

---

SELECT [Group], MIN([Min_ID]), MAX([Max_ID])
FROM (
  SELECT [score].[Group], [Min_ID] = [min].ID, [Max_ID] = [max].ID
  FROM (
    SELECT [Group], [Min_Score] = MIN(Score), [Max_Score] = MAX(Score)
    FROM @YourTable
    GROUP BY [GROUP]) score
    INNER JOIN @YourTable [min] ON [min].[Group] = [score].[Group] AND [min].[Score] = [score].[Min_Score]
    INNER JOIN @YourTable [max] ON [max].[Group] = [score].[Group] AND [max].[Score] = [score].[Max_Score] 
  ) yourtable
GROUP BY [yourtable].[Group]

Answer 3

如果可能，最好有一个通用的解决方案，让我可以选择每个组的顶部和底部 n 值。

WITH q AS
        (
        SELECT  m.*,
                ROW_NUMBER() OVER (PARTITION BY Group ORDER BY Score) AS rn_asc,
                ROW_NUMBER() OVER (PARTITION BY Group ORDER BY Score DESC) AS rn_desc
        FROM    mytable m
        )
SELECT  *
FROM    q
WHERE   rn_asc BETWEEN 1 AND 10
        OR rn_desc BETWEEN 1 AND 10

Answer 4

任何解决方案还需要一些关于组和分数的良好索引，但包括 ID

SELECT
    foo.[Group],
    m1.ID AS Min_ID,
    m2.ID AS Max_ID
FROM
    (
    SELECT
       [Group], MIN(Score) AS MinScore, MAX(Score) AS MaxScore
    FROM
       mytable
    GROUP BY
       [Group]
    ) foo
    JOIN
    mytable m1 ON foo.[Group] = m1.[Group] AND foo.MinScore = m1.Score
    JOIN
    mytable m2 ON foo.[Group] = m2.[Group] AND foo.MaxScore = m2.Score

但是，在您的示例数据中，这也有效，因为 ID 和分数是按顺序对齐的：

SELECT
    [Group],
    MIN(ID) AS Min_ID,
    MAX(ID) AS Max_ID
FROM
    mytable
GROUP BY
    [Group]