sql - Selecting first row per group

Question

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

The values are:

+---+-----+----+
| 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 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?

So, this should result in:

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

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) for the queries to work faster.

Which one is more efficient depends on your data distribution.

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

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