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Selecting the first row for each group

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

Values:

+---+-----+----+
| 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 |
+---+-----+----+

If I order data first by column A, then column B, then column C(although I made all the column values ​​the Csame in the column A), how would I select the “first row” for the column A?

So this should result in:

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

or

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

Create a composite index on (a, b, c)to make queries faster.

Which one is more efficient depends on your data distribution.

If you have several different values a, but many entries in each a, a second query would be better.

, :

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
+14

,

SQL-?

GROUP BY?

" sql"

+1
SELECT *
FROM
  (SELECT *,
          ROW_NUMBER() OVER (PARTITION BY Dealld
                             ORDER BY Price, Dealld) rn
   FROM DealOffers) q
WHERE rn = 1
ORDER BY Name
0
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