选择每组的第一行 [英] Selecting first row per group

问题描述

我有一个包含三列的表格:A,B,C.

I have a table with three columns: A,B,C.

值为:

+---+-----+----+
| A |  B  | C  |
+---+-----+----+
| 1 | -10 |  5 |
| 1 |   0 |  5 |
| 1 |  10 |  5 |
| 2 |  10 | 12 |
| 2 |   0 | 12 |
| 3 | -10 | 14 |
| 4 |   0 |  8 |
| 4 |  10 |  8 |
| 5 |   0 |  6 |
| 5 |   1 |  6 |
| 5 |  -5 |  6 |
+---+-----+----+

如果我首先按 A 列对数据进行排序，然后是 B 列，然后是 C 列(尽管我确实使所有列 >C 每列的值相同 A 值)我将如何选择每列 A 的第一行"?

If I first order the data by column A, then column B, then column C (although I did make all column C values the same per column A value) how would I select the "first row" per column A?

所以，这应该导致:

+---+-----+----+
| A |  B  | C  |
+---+-----+----+
| 1 | -10 |  5 |
| 2 |   0 | 12 |
| 3 | -10 | 14 |
| 4 |   0 |  8 |
| 5 |  -5 |  6 |
+---+-----+----+

推荐答案

SELECT  a, b, c
FROM    (
        SELECT  *, ROW_NUMBER() OVER (PARTITION BY a ORDER BY b, c) rn
        FROM    mytable
        ) q
WHERE   rn = 1
ORDER BY
        a

或

SELECT  mi.*
FROM    (
        SELECT  DISTINCT  a
        FROM    mytable
        ) md
CROSS APPLY
        (
        SELECT  TOP 1 *
        FROM    mytable mi
        WHERE   mi.a = md.a
        ORDER BY
                b, c
        ) mi
ORDER BY
        a

在 (a, b, c) 上创建一个复合索引，以便查询更快地工作.

Create a composite index on (a, b, c) for the queries to work faster.

哪个更有效取决于您的数据分布.

Which one is more efficient depends on your data distribution.

如果 a 的不同值很少，但每个 a 中有很多记录，那么第二个查询会更好.

If you have few distinct values of a but lots of records within each a, the second query would be better.

您可以通过创建索引视图进一步改进它:

You could improve it even more by creating an indexed view:

CREATE VIEW v_mytable_da
WITH   SCHEMABINDING
AS
       SELECT  a, COUNT_BIG(*) cnt
       FROM    dbo.mytable
       GROUP BY
               a

GO

CREATE UNIQUE CLUSTERED INDEX
       pk_vmytableda_a
ON     v_mytable_da (a)

GO

SELECT  mi.*
FROM    v_mytable_da md
CROSS APPLY
        (
        SELECT  TOP 1 *
        FROM    mytable mi
        WHERE   mi.a = md.a
        ORDER BY
                b, c
        ) mi
ORDER BY
        a

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