t-sql查询：检索最新的行方法

Based on the following table

基于下表

Table_A

ID   Rev  Description
-----------------------------------
1    1    Some text.
1    2    Some text. Adding more.
1    3    Some text. Ading more & more.

The above will keep adding a new row when user updates the description.

当用户更新描述时，上面将继续添加新行。

I want to take the row with MAX(Rev) [i.e. the latest description].

我想用MAX（Rev）取行[即最新描述]。

To get this I do the following:

为此，我执行以下操作：

;with AllDescriptions As
(
 select 
        ID
        , Rev
        , Description 
        , ROW_NUMBER() over (partition by ID order by Rev desc) as RowNum
        from Table_A
        Where ID = 1
)
select ID, Rev, Description from AllDescription
where RowNum = 1

Recently I saw a different approach to getting the same result

最近我看到了一种不同的方法来获得相同的结果

select b.* from 
(
 select ID, MAX(Rev) as MaxRev 
 from Table_A 
 where ID = 1
 group by ID
) as a
inner join 
(
 select ID, Rev, Description from Table_A where ID = 1
) as b
on a.ID = b.ID and a.MaxRev = b.Rev

From learning perspective, I want to know Which of the above two approaches is better? Or if there is even better way to do the same?

从学习的角度来看，我想知道上述哪两种方法更好？或者，如果有更好的方法来做同样的事情？

3 个解决方案

#1

The second approach looks like a SQL Server 2000 approach before Row_Number() was introduced. This is the Greatest-n-per-group problem.

在引入Row_Number（）之前，第二种方法看起来像SQL Server 2000方法。这是每组最大的问题。

To evaluate them you should look at the execution plans and the I/O Stats by running SET STATISTICS IO ON

要评估它们，您应该通过运行SET STATISTICS IO ON来查看执行计划和I / O统计信息

Of course for the specific example you have given the following would work equally well

当然，对于您给出以下内容的具体示例，同样可以正常工作

 select TOP 1
        ID
        , Rev
        , Description 
        from Table_A
        Where ID = 1
ORDER BY Rev desc

#2

If you have a parent table with each ID listed only once, this can sometimes outperform other strategies including the row_number solution:

如果您有一个父表，每个ID只列出一次，这有时可以胜过其他策略，包括row_number解决方案：

SELECT
   X.*
FROM
   ParentTable P
   CROSS APPLY (
      SELECT TOP 1 *
      FROM Table_A A
      WHERE P.ID = A.ID
      ORDER BY A.Rev DESC
   ) X

And the crazy, patented (just kidding), single scan magic query which can often outperform other methods as well:

而疯狂的，专利的（只是开玩笑），单扫描魔术查询，通常也可以胜过其他方法：

SELECT
   ID,
   Rev = Convert(int, Substring(Packed, 1, 4)),
   Description = Convert(varchar(100), Substring(Packed, 5, 100))
FROM
   (
      SELECT
         ID,
         Packed = Max(Convert(binary(4), Rev) + Convert(varbinary(100), Description))
      FROM Table_A
      GROUP BY ID
   ) X

Note: this last method is not recommended, but it's fun to simulate the First/Last aggregates in MS Access.

注意：不推荐使用最后一种方法，但模拟MS Access中的First / Last聚合很有趣。

#3

I would tend to favour the first approach - from a readability perspective, once you're comfortable with the ROW_NUMBER() OVER ... syntax then it's somewhat more readable. From a performance perspective, I would be surprised if there was much difference between the two - if there is then I would expect that the second would perform worse - but I stand to be corrected on that!

我倾向于赞成第一种方法 - 从可读性的角度来看，一旦你对ROW_NUMBER（）OVER ...语法感到满意，那么它就会更具可读性。从性能的角度来看，如果两者之间存在很大差异，我会感到惊讶 - 如果有的话，我会期望第二个会表现更差 - 但我会立即予以纠正！

#1