Microsoft SQL Server选择前N个组

Question

关于如何从每个组中选择n行有很多答案。 但是我要寻找的是从前N个组中选择每一行，例如，我有以下数据：

 id   group
  1     a
  2     a
  3     b
  4     c
  5     c
  6     d
  7     d
  .......

如果要选择前三名，我的预期结果如下：

   1    a
   2    a
   3    b
   4    c
   5    c

如何使用Microsoft SQL Server 2008做到这一点？

Answer 1

一种选择是使用子查询来选择前N个组：

SELECT t1.id, t1.group
FROM yourTable t1
INNER JOIN
(
    SELECT DISTINCT TOP(N) group
    FROM yourTable
    ORDER BY group
) t2
    ON t1.group = t2.group

Answer 2

您可以按组对行进行rank ，然后仅采用前三名：

SELECT [id], [group]
FROM   (SELECT [id], [group], RANK() OVER (ORDER BY [group] ASC) rk
        FROM   mytable) t
WHERE  rk <= 3

Answer 3

@Tim：我刚刚修改了您的查询。

SELECT t1.id, t1.group
FROM yourTable t1
INNER JOIN
(
    SELECT TOP N group
    FROM yourTable
    GROUP BY group
    --ORDER BY group USE IT IF YOU WANT
) t2
    ON t1.group = t2.group