sql - 选择每组的第一行

Question

我有一个包含三列的表：A,B,C.

这些值为：

+---+-----+----+
| 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 column A，然后 column B，然后 column对数据进行排序C（尽管我确实使C每列值的所有列值都相同A）我将如何选择每列的“第一行” A？

因此，这应该导致：

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

Answer 1

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

或者

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

为查询创建一个复合索引(a, b, c)以更快地工作。

哪个更有效取决于您的数据分布。

如果a每个 的不同值很少但有很多记录a，则第二个查询会更好。

您可以通过创建索引视图来进一步改进它：

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

Answer 2

SELECT *
FROM
  (SELECT *,
          ROW_NUMBER() OVER (PARTITION BY Dealld
                             ORDER BY Price, Dealld) rn
   FROM DealOffers) q
WHERE rn = 1
ORDER BY Name