选择 group by 子句中的前 n 行 [英] Selecting the top n rows within a group by clause

问题描述

我的架构类似于以下内容:

I have schema similar to the following:

create table bar
(
    instrument varchar(255) not null,
    bar_dttm datetime not null,
    bar_open int not null,
    bar_close int not null
)

我想查询表格，并返回每个工具最近的 5 行.

I would like to query the table, and return the most recent 5 rows per instrument.

我可以逐个乐器地进行，使用:

I can do it instrument by instrument, with:

select top 5 instrument, bar_dttm, bar_open, bar_close
from bar
where instrument = 'XXX'
order by bar_dttm desc

我想在一个查询中一次为所有仪器执行此操作.这可能吗?我正在运行 SQL Server 2008.

I'd like to do this for all instruments at once in one query. Is this possible? I am running SQL Server 2008.

推荐答案

CROSS APPLY 是您通常执行此操作的方式 - http://msdn.microsoft.com/en-us/library/ms175156.aspx

CROSS APPLY is how you usually do this - http://msdn.microsoft.com/en-us/library/ms175156.aspx

编辑 - 添加示例，如下所示:

EDIT - add example, something like this:

select
    bar1.instrument
    ,bar2.*
from (
    select distinct instrument from bar) as bar1
cross apply (
    select top 5
        bar2.instrument
        ,bar2.bar_dttm
        ,bar2.bar_open
        ,bar2.bar_close 
    from bar as bar2 where bar2.instrument = bar1.instrument) as bar2

通常您希望在那里添加订单.

Typically you would want to add an order by in there.

编辑 - 为查询添加了不同的内容，希望能满足您的需求.编辑 - 在顶部添加了缺少的选择"关键字.复制&粘贴错误 FTL！

Edit - added distinct to the query, hopefully that gives you want you want. Edit - added missing 'select' keyword at top. copy & paste bug FTL!

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