如果你想生成按节分组的输出，只显示每个节的前n条记录，如下所示:

SECTION     SUBSECTION

deer        American Elk/Wapiti
deer        Chinese Water Deer
dog         Cocker Spaniel
dog         German Shephard
horse       Appaloosa
horse       Morgan

.．.那么下面的代码应该适用于所有SQL数据库。如果您想要前10，只需在查询的末尾将2更改为10。

select
    x1.section
    , x1.subsection
from example x1
where
    (
    select count(*)
    from example x2
    where x2.section = x1.section
    and x2.subsection <= x1.subsection
    ) <= 2
order by section, subsection;

设置:

create table example ( id int, section varchar(25), subsection varchar(25) );

insert into example select 0, 'dog', 'Labrador Retriever';
insert into example select 1, 'deer', 'Whitetail';
insert into example select 2, 'horse', 'Morgan';
insert into example select 3, 'horse', 'Tarpan';
insert into example select 4, 'deer', 'Row';
insert into example select 5, 'horse', 'Appaloosa';
insert into example select 6, 'dog', 'German Shephard';
insert into example select 7, 'horse', 'Thoroughbred';
insert into example select 8, 'dog', 'Mutt';
insert into example select 9, 'horse', 'Welara Pony';
insert into example select 10, 'dog', 'Cocker Spaniel';
insert into example select 11, 'deer', 'American Elk/Wapiti';
insert into example select 12, 'horse', 'Shetland Pony';
insert into example select 13, 'deer', 'Chinese Water Deer';
insert into example select 14, 'deer', 'Fallow';

如果我们使用SQL Server >= 2005，那么我们可以只用一个选择来解决任务:

declare @t table (
    Id      int ,
    Section int,
    Moment  date
);

insert into @t values
(   1   ,   1   , '2014-01-01'),
(   2   ,   1   , '2014-01-02'),
(   3   ,   1   , '2014-01-03'),
(   4   ,   1   , '2014-01-04'),
(   5   ,   1   , '2014-01-05'),

(   6   ,   2   , '2014-02-06'),
(   7   ,   2   , '2014-02-07'),
(   8   ,   2   , '2014-02-08'),
(   9   ,   2   , '2014-02-09'),
(   10  ,   2   , '2014-02-10'),

(   11  ,   3   , '2014-03-11'),
(   12  ,   3   , '2014-03-12'),
(   13  ,   3   , '2014-03-13'),
(   14  ,   3   , '2014-03-14'),
(   15  ,   3   , '2014-03-15');


-- TWO earliest records in each Section

select top 1 with ties
    Id, Section, Moment 
from
    @t
order by 
    case 
        when row_number() over(partition by Section order by Moment) <= 2 
        then 0 
        else 1 
    end;


-- THREE earliest records in each Section

select top 1 with ties
    Id, Section, Moment 
from
    @t
order by 
    case 
        when row_number() over(partition by Section order by Moment) <= 3 
        then 0 
        else 1 
    end;


-- three LATEST records in each Section

select top 1 with ties
    Id, Section, Moment 
from
    @t
order by 
    case 
        when row_number() over(partition by Section order by Moment desc) <= 3 
        then 0 
        else 1 
    end;

如果你正在使用SQL 2005，你可以这样做…

SELECT rs.Field1,rs.Field2 
    FROM (
        SELECT Field1,Field2, Rank() 
          over (Partition BY Section
                ORDER BY RankCriteria DESC ) AS Rank
        FROM table
        ) rs WHERE Rank <= 10

如果你的RankCriteria有平局，那么你可能会返回超过10行，Matt的解决方案可能更适合你。

您可以尝试这种方法。 这个查询为每个国家返回10个人口最多的城市。

   SELECT city, country, population
   FROM
   (SELECT city, country, population, 
   @country_rank := IF(@current_country = country, @country_rank + 1, 1) AS country_rank,
   @current_country := country 
   FROM cities
   ORDER BY country, population DESC
   ) ranked
   WHERE country_rank <= 10;

如果你想生成按节分组的输出，只显示每个节的前n条记录，如下所示:

SECTION     SUBSECTION

deer        American Elk/Wapiti
deer        Chinese Water Deer
dog         Cocker Spaniel
dog         German Shephard
horse       Appaloosa
horse       Morgan

.．.那么下面的代码应该适用于所有SQL数据库。如果您想要前10，只需在查询的末尾将2更改为10。

select
    x1.section
    , x1.subsection
from example x1
where
    (
    select count(*)
    from example x2
    where x2.section = x1.section
    and x2.subsection <= x1.subsection
    ) <= 2
order by section, subsection;

设置:

create table example ( id int, section varchar(25), subsection varchar(25) );

insert into example select 0, 'dog', 'Labrador Retriever';
insert into example select 1, 'deer', 'Whitetail';
insert into example select 2, 'horse', 'Morgan';
insert into example select 3, 'horse', 'Tarpan';
insert into example select 4, 'deer', 'Row';
insert into example select 5, 'horse', 'Appaloosa';
insert into example select 6, 'dog', 'German Shephard';
insert into example select 7, 'horse', 'Thoroughbred';
insert into example select 8, 'dog', 'Mutt';
insert into example select 9, 'horse', 'Welara Pony';
insert into example select 10, 'dog', 'Cocker Spaniel';
insert into example select 11, 'deer', 'American Elk/Wapiti';
insert into example select 12, 'horse', 'Shetland Pony';
insert into example select 13, 'deer', 'Chinese Water Deer';
insert into example select 14, 'deer', 'Fallow';

Q)从每个组中找到TOP X记录(Oracle)

SQL> select * from emp e 
  2  where e.empno in (select d.empno from emp d 
  3  where d.deptno=e.deptno and rownum<3)
  4  order by deptno
  5  ;

 EMPNO ENAME      JOB              MGR HIREDATE         SAL       COMM     DEPTNO

  7782 CLARK      MANAGER         7839 09-JUN-81       2450                    10
  7839 KING       PRESIDENT            17-NOV-81       5000                    10
  7369 SMITH      CLERK           7902 17-DEC-80        800                    20
  7566 JONES      MANAGER         7839 02-APR-81       2975                    20
  7499 ALLEN      SALESMAN        7698 20-FEB-81       1600        300         30
  7521 WARD       SALESMAN        7698 22-FEB-81       1250        500         30

选定6行。