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03.01.2008 at 12:26PM PST, ID: 23206573

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9.2

SQL without inner join/subselect

Zones: SQL Query Syntax, Databases Miscellaneous, Oracle Database

Tags: Oracle, Oracle Database, 10g, SQL

I'm thinking of a solution for following query without explicit inner join or subquery:

Table FXR:
FX_Code|FX_Rate|FX_Date
'USD',1.15,'2008-01-01'
'USD',1.2,'2008-02-01'
'EUR',1.1,'2008-01-15'
'EUR',1.5, '2008-01-17'

I want a query that gives me the latest FX_Rate,FX_Date for each FX_Code:
'USD',1.2,'2008-02-01'
'EUR',1.5,'2008-01-17'

this must be simpler than doing this with a subquery:
select FX_Code,FX_Rate,FX_Date from FXR
where (FX_Code,FX_Date) in (
select FX_Code,max(FX_Date) from FXR
group by FX_Code
)

thanks for any hint
michi

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Question Stats

Zone: Programming

Question Asked By: witty

Solution Provided By: sdstuber

Participating Experts: 4

Solution Grade: A

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03.01.2008 at 01:18PM PST, ID: 21023586

Rank: Guru

harfang:

This question ("find the earliest/latest record for each group") does require a subquery, and as far as I know there is no "simpler" method.

However, depending on the database engine, using Max() and a GROUP BY query might not be the most optimal. I often use on of the following instead. I'm assuming that (FX_Code; FX_Rate) is the key of the table FXR.

select FX_Code,FX_Rate,FX_Date from FXR
where not exists (
select 1 from FXR Z
where FX_Code = FXR.FX_Code And FX_Date > FXR.FX_Date
)

[Access syntax don't know the Oracle equivalent of DISTINCTROW:]
select distinctrow FX_Code,FX_Rate,FX_Date
from FXR left join FXR Z
on FXR.FX_Code = Z.FX_Code And FXR.FX_Date < Z.FX_Date
where Z.FX_Code Is Null

select FX_Code,FX_Rate,FX_Date from FXR
where FX_Date = (
select top 1 FX_Date from FXR Z
where Z.FX_Code = FXR.FX_Code
)

As you see, not "simpler" at all.

Cheers!
(°v°)

03.01.2008 at 01:23PM PST, ID: 21023612

Rank: Guru

harfang:

Sorry, forgot the *don't*. I often see this query, which is perhaps what you were hoping for:

select FX_Code, Last(FX_Rate), Last(FX_Date)
from FXR
group by FX_Code

This even works e.g. in Access on a freshly compacted database, because the table is resorted and stored in the key order. So Last() does indeed mean "latest". However, unless your database engine honors an ORDER BY clause prior to grouping, the results are unpredictable.

select FX_Code, Last(FX_Rate), Last(FX_Date)
from FXR
"order by before grouping" FX_Code, FX_Date
group by FX_Code

Cheers!
(°v°)

03.01.2008 at 01:24PM PST, ID: 21023615

Rank: Sage

sdstuber:

You definitely don't want to do it with a join or a sub-query to the same table again.
You will have to use analytics though but the subquery is quite efficient and (in my opinion) simple to read

SELECT fx_code, fx_rate, fx_date
FROM (SELECT fx_code, fx_rate, fx_date,
ROW_NUMBER() OVER(PARTITION BY fx_code ORDER BY fx_date DESC)
rn
FROM fxr)
WHERE rn = 1

Accepted Solution

03.01.2008 at 01:30PM PST, ID: 21023656

JasonRawlins:

Is there a reason why you don't want to use a sub query or inner join? SQL Server 2005's TSQL syntax has RANK() function that can do this, but I can't do it in just SQL.

I'll post the SQL Server function in case Oracle has something similar.

                       
             SELECT FX_Code, FX_Date, FX_Rate
FROM 
(
	SELECT FX_Code, FX_Date, FX_Rate, RANK() OVER (PARTITION BY FX_Code ORDER BY FX_Date DESC) AS Rank
	FROM @FXR AS fxr
) AS tmp
WHERE Rank = 1

Open in New Window

03.01.2008 at 02:25PM PST, ID: 21023917

Akenathon:

The question is interesting, since intuitively you *should* be right! :-)

However, if you think in set algebra, you need to do this:

- First, go through all rows grouped by code, and determine which are the ones that have the max(date) for each equivalence class
- Then, revisit those rows to get some other fields' data (you can make it faster by using rowid access, but you'll still make a second pass)

Now you'll rightfully say... why can't you get my rates when you make the groups? I suppose it's just a SQL limitation: you can only project either grouped things, or aggregates on non-grouped things :-/

I've included the ROWID trick to ease the pain. Just for the record: you could try using analytics' MAX(FX_DATE) over(partition by FX_CODE), but that will also make another pass -look at the execution plan and you'll see...

                       
             select FX_Code,FX_Rate,FX_Date from FXR
where (rowid,FX_Code,FX_Date) in (
  select rowid,FX_Code,max(FX_Date) from FXR
  group by FX_Code
)

Open in New Window

03.01.2008 at 02:50PM PST, ID: 21024022

Rank: Sage

sdstuber:

I would definitely avoid the requery to FXR, even with rowid's. Imagine if FXR has a lot of data. Even rowid access isn't fast enough.
Just use analytics, this kind of query is exactly what they were built for

03.01.2008 at 05:44PM PST, ID: 21024574

witty:

@Akenathon:
but wouldn't I have to include rowid also in the groupby?
Otherwise I would get the a record-specific result with a aggregated value -> exactly what I need, but what I would not expect here to work ...

@sdstuber:
sounds logical to me - but internally oracle has to do "something like self join" (perhaps more performant) - right?

thx michi

03.01.2008 at 09:14PM PST, ID: 21025050

Rank: Sage

sdstuber:

No, analytics don't use self joins. At least not in this case. It uses a sort.

In this case it's sorting by fx_code, fx_date.

Do an explain plan on the query and you'll see a sort step included.
That's how it finds which row comes first within each fx_code

Then it's just a matter keeping only the first row of each fx_code.

Assisted Solution

03.02.2008 at 06:33AM PST, ID: 21026071

Akenathon:

Yes, you are right... you can't do the rowid thing without another access/join :-/

Now, the analytics won't do any self join. Analytics are applied after the result set is determined, partitioning+sorting+calculating the funcion in memory. So, it will make another pass, but not one which needs another access to the blocks. You'll see it as "WINDOW SORT" in your execution plan.

I think I'd go with sdstuber's solution for this one... it makes one pass accessing the blocks, then one pass in memory for grouping (i.e. "partitioning") and sorting. The only weak scenario is when each partition (i.e. group) is very large: I don't think the CBO is clever enough to propagate the "outer" predicate rn=1 to the subquery, so other analytics like MAX and FIRST_VALUE could also be explored... no time now, but I'll check this thread when I'm back :-)

03.02.2008 at 07:44AM PST, ID: 21026346

Rank: Sage

sdstuber:

actually you "can" get "stopkey" steps in your explain plan.
Where the optimizer decides to push the predicate, but when it will do that and when it won't aren't something I know how to determine.

20080236-EE-VQP-29 / EE_QW_2_20070628