Question

Based on the following table

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.

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

To get this I do the following:

;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

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?

Answer 1

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

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

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

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