Обсуждение: oracle's first_value function for postgres?

Oracle has a very handy function called first_value, which can be used
to turn a set like this:

a   10
a   3
b   1
c   30
c   10
d   1
d   20

...into this:

a   10
b   1
c   30
d   1

Does postgres have something equivalent, or, even better, is there a
reasonable way to express this in standard SQL?

* Ben (bench@silentmedia.com) wrote:
> Oracle has a very handy function called first_value, which can be used
> to turn a set like this:
>
> a   10
> a   3
> b   1
> c   30
> c   10
> d   1
> d   20
>
> ...into this:
>
> a   10
> b   1
> c   30
> d   1
>
> Does postgres have something equivalent, or, even better, is there a
> reasonable way to express this in standard SQL?

Isn't this just: distinct on (col1)?

    Stephen

Sweet, indeed it is. DISTINCT ON seems to be a postgresism, but I can
live with that.

Thanks.

Stephen Frost wrote:

>
>Isn't this just: distinct on (col1)?
>
>    Stephen
>
>

On Tue, Sep 13, 2005 at 12:15:21PM -0700, Ben wrote:
> Oracle has a very handy function called first_value, which can be used
> to turn a set like this:

Look at DISTINCT ON ()

> Does postgres have something equivalent, or, even better, is there a
> reasonable way to express this in standard SQL?

In standard SQL, not really, which is why it's an extention...
--
Martijn van Oosterhout   <kleptog@svana.org>   http://svana.org/kleptog/
> Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a
> tool for doing 5% of the work and then sitting around waiting for someone
> else to do the other 95% so you can sue them.

Martijn van Oosterhout <kleptog@svana.org> writes:
> On Tue, Sep 13, 2005 at 12:15:21PM -0700, Ben wrote:
>> Oracle has a very handy function called first_value, which can be used
>> to turn a set like this:

> Look at DISTINCT ON ()

>> Does postgres have something equivalent, or, even better, is there a
>> reasonable way to express this in standard SQL?

> In standard SQL, not really, which is why it's an extention...

Another way is to create an aggregate with a no-op transition
function ... also nonstandard, but you might be able to duplicate
Oracle's syntax that way.

            regards, tom lane

* Ben (bench@silentmedia.com) wrote:
> Sweet, indeed it is. DISTINCT ON seems to be a postgresism, but I can
> live with that.

Uhhh, I was pretty sure it was standard SQL...  I'd have to go look it
up though, to be sure.

Don't forget to order by to make sure you get something consistent. :)

    Enjoy,

        Stephen

Stephen Frost <sfrost@snowman.net> writes:
> * Ben (bench@silentmedia.com) wrote:
>> Sweet, indeed it is. DISTINCT ON seems to be a postgresism, but I can
>> live with that.

> Uhhh, I was pretty sure it was standard SQL...

Nope, definitely a Postgres-ism.

            regards, tom lane

* Tom Lane (tgl@sss.pgh.pa.us) wrote:
> Stephen Frost <sfrost@snowman.net> writes:
> > * Ben (bench@silentmedia.com) wrote:
> >> Sweet, indeed it is. DISTINCT ON seems to be a postgresism, but I can
> >> live with that.
>
> > Uhhh, I was pretty sure it was standard SQL...
>
> Nope, definitely a Postgres-ism.

Huh.  Guess I suck then.  I actually do this a fair bit, is there any
way to do it in standard SQL?  It's terribly useful...

    Thanks,

        Stephen

Martijn van Oosterhout wrote:
> On Tue, Sep 13, 2005 at 12:15:21PM -0700, Ben wrote:
>
>>Oracle has a very handy function called first_value, which can be used
>>to turn a set like this:
>
>
> Look at DISTINCT ON ()
>
>
>>Does postgres have something equivalent, or, even better, is there a
>>reasonable way to express this in standard SQL?
>
>
> In standard SQL, not really, which is why it's an extention...

No?    It seems this is similar

SELECT a,b
FROM ( SELECT a,b,
    RANK() OVER (
        PARTITION BY a
        ORDER BY b
    ) rank
    FROM my_table )
WHERE rank = 1
ORDER BY a, rank DESC;

which I think is standard sql-99 with the sql-99 olap extention.