Обсуждение: selecting latest record

Hi,

I have a simple table 

price(id_product, price, date) 

which records price changes for each id_product. Each time a price
changes a new tuple is created.

What is the best way to select only the latest price of each id_product?

Thanks,

Hello

2009/9/22 Louis-David Mitterrand <vindex+lists-pgsql-sql@apartia.org>:
> Hi,
>
> I have a simple table
>
> price(id_product, price, date)
>
> which records price changes for each id_product. Each time a price
> changes a new tuple is created.
>
> What is the best way to select only the latest price of each id_product?

there are more ways - depends on what you wont.

one way is

SELECT *  FROM price WHERE (id_product, date) = (SELECT id_product, max(date)
                  FROM price                                                             GROUP BY
 
id_product)

Regards
Pavel Stehule

>
> Thanks,
>
> --
> Sent via pgsql-sql mailing list (pgsql-sql@postgresql.org)
> To make changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-sql
>

On Tue, Sep 22, 2009 at 11:56:54AM +0200, Pavel Stehule wrote:
> 
> there are more ways - depends on what you wont.
> 
> one way is
> 
> SELECT *
>    FROM price
>   WHERE (id_product, date) = (SELECT id_product, max(date)
>                                                                FROM price
>                                                               GROUP BY
> id_product)

Nice. 

I didn't know one could have several args in a single WHERE clause.

Thanks,

In response to Louis-David Mitterrand :
> Hi,
> 
> I have a simple table 
> 
> price(id_product, price, date) 
> 
> which records price changes for each id_product. Each time a price
> changes a new tuple is created.
> 
> What is the best way to select only the latest price of each id_product?

There are several ways to do that, for instance with DISTINCT ON (only
postgresql):

test=*# select * from price ;id_product | price |   datum
------------+-------+------------         1 |    10 | 2009-09-01         1 |    12 | 2009-09-10         2 |    11 |
2009-09-10        2 |     8 | 2009-09-13
 
(4 rows)

test=*# select distinct on (id_product) id_product, price from price order by id_product, datum desc;id_product |
price
------------+-------         1 |    12         2 |     8
(2 rows)

Andreas
-- 
Andreas Kretschmer
Kontakt:  Heynitz: 035242/47150,   D1: 0160/7141639 (mehr: -> Header)

Let's say there's an index on the date column: Does the where clause 
approach necessarily out perform the distinct on version? Hoping the OP 
has enough data to make analyse useful.

A. Kretschmer wrote:
> In response to Louis-David Mitterrand :
>   
>> Hi,
>>
>> I have a simple table 
>>
>> price(id_product, price, date) 
>>
>> which records price changes for each id_product. Each time a price
>> changes a new tuple is created.
>>
>> What is the best way to select only the latest price of each id_product?
>>     
>
> There are several ways to do that, for instance with DISTINCT ON (only
> postgresql):
>
> test=*# select * from price ;
>  id_product | price |   datum
> ------------+-------+------------
>           1 |    10 | 2009-09-01
>           1 |    12 | 2009-09-10
>           2 |    11 | 2009-09-10
>           2 |     8 | 2009-09-13
> (4 rows)
>
> test=*# select distinct on (id_product) id_product, price from price order by id_product, datum desc;
>  id_product | price
> ------------+-------
>           1 |    12
>           2 |     8
> (2 rows)
>
> Andreas
>