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Thread: get first and last row in one sql as two columns

get first and last row in one sql as two columns

From
Tom Smith
Date:
Hi:

I need to get the first and last tow in one sql like below

select first(col1), last(col1) from table order by col1

I saw some posting in wiki with a custom function (or C extention)
to do this.   Is it widely used and reliable?
https://wiki.postgresql.org/wiki/First/last_(aggregate)

I am wondering why these two functions are not part of postgresql built-in
functions as it has many use cases

Thanks

Re: get first and last row in one sql as two columns

From
Rob Sargent
Date:
On 09/02/2015 05:14 PM, Tom Smith wrote:
Hi:

I need to get the first and last tow in one sql like below

select first(col1), last(col1) from table order by col1

I saw some posting in wiki with a custom function (or C extention)
to do this.   Is it widely used and reliable?
https://wiki.postgresql.org/wiki/First/last_(aggregate)

I am wondering why these two functions are not part of postgresql built-in
functions as it has many use cases

Thanks
If you're ordering by col1, does
select min(col1), max(col1) from table order by col1

not do the trick;

Re: get first and last row in one sql as two columns

From
Rob Sargent
Date:
On 09/02/2015 05:14 PM, Tom Smith wrote:
Hi:

I need to get the first and last tow in one sql like below

select first(col1), last(col1) from table order by col1

I saw some posting in wiki with a custom function (or C extention)
to do this.   Is it widely used and reliable?
https://wiki.postgresql.org/wiki/First/last_(aggregate)

I am wondering why these two functions are not part of postgresql built-in
functions as it has many use cases

Thanks
But what I think you are looking for are the windowing functions as what you propose breaks down pretty quickly with more complicated queries.

Re: get first and last row in one sql as two columns

From
Melvin Davidson
Date:
Try this:

SELECT
  (SELECT <your_column>
     FROM <your_table>
     ORDER BY <your_column> offset 0 LIMIT 1) ,
  (SELECT <your_column>
     FROM <your_table>
     ORDER BY <your_column> OFFSET (SELECT COUNT(*) ) LIMIT 1)
   FROM <your_table> LIMIT 1;


On Wed, Sep 2, 2015 at 7:27 PM, Rob Sargent <robjsargent@gmail.com> wrote:
On 09/02/2015 05:14 PM, Tom Smith wrote:
Hi:

I need to get the first and last tow in one sql like below

select first(col1), last(col1) from table order by col1

I saw some posting in wiki with a custom function (or C extention)
to do this.   Is it widely used and reliable?
https://wiki.postgresql.org/wiki/First/last_(aggregate)

I am wondering why these two functions are not part of postgresql built-in
functions as it has many use cases

Thanks
If you're ordering by col1, does
select min(col1), max(col1) from table order by col1

not do the trick;



--
Melvin Davidson
I reserve the right to fantasize.  Whether or not you
wish to share my fantasy is entirely up to you.

Re: get first and last row in one sql as two columns

From
Tom Smith
Date:
Thanks for suggestion.  I could use two separate sqls, but I am looking to get it in a single sql.
I just compiled and installed (under postgres id)  first_last_agg C extension.
http://pgxn.org/dist/first_last_agg/
it WORKED under postgres user with sample
select first(column1), last(column1) from (values (null),(1),(3),(null)) as x
but somehow when I run under other users, it say

ERROR:  function first(integer) does not exist

below is the control file ,  I can not find out why I can not run under other user ids.
Any help would be appreciated.

# first_last_agg extension
comment = 'first() and last() aggregate functions'
default_version = '0.1.4'
module_pathname = '$libdir/first_last_agg'
relocatable = false
schema = pg_catalog
superuser = false







On Wed, Sep 2, 2015 at 7:47 PM, Melvin Davidson <melvin6925@gmail.com> wrote:
Try this:

SELECT
  (SELECT <your_column>
     FROM <your_table>
     ORDER BY <your_column> offset 0 LIMIT 1) ,
  (SELECT <your_column>
     FROM <your_table>
     ORDER BY <your_column> OFFSET (SELECT COUNT(*) ) LIMIT 1)
   FROM <your_table> LIMIT 1;


On Wed, Sep 2, 2015 at 7:27 PM, Rob Sargent <robjsargent@gmail.com> wrote:
On 09/02/2015 05:14 PM, Tom Smith wrote:
Hi:

I need to get the first and last tow in one sql like below

select first(col1), last(col1) from table order by col1

I saw some posting in wiki with a custom function (or C extention)
to do this.   Is it widely used and reliable?
https://wiki.postgresql.org/wiki/First/last_(aggregate)

I am wondering why these two functions are not part of postgresql built-in
functions as it has many use cases

Thanks
If you're ordering by col1, does
select min(col1), max(col1) from table order by col1

not do the trick;



--
Melvin Davidson
I reserve the right to fantasize.  Whether or not you
wish to share my fantasy is entirely up to you.

Re: get first and last row in one sql as two columns

From
"Dickson S. Guedes"
Date:
On Wed, Sep 02, 2015 at 07:14:40PM -0400, Tom Smith wrote:
> Hi:
>
> I need to get the first and last tow in one sql like below
>
> select first(col1), last(col1) from table order by col1

Have you tried the window functions [1] last_value and first_value?

You could use something like:

SELECT first_value(col1) over (order by col1),
       last_value(col1)  over (order by col1)
FROM table;

It should be as simple as that, or I miss your point.


[1] http://www.postgresql.org/docs/current/static/functions-window.html


[]s
--
Dickson S. Guedes
@guediz - http://github.com/guedes
Attachment

Re: get first and last row in one sql as two columns

From
Tom Smith
Date:
Hi:

The window function works for me (with adding  limit 1 in the end to output only one row
 needed instead of many duplicate rows).

thanks very much. 

On Thu, Sep 3, 2015 at 6:51 AM, Dickson S. Guedes <listas@guedesoft.net> wrote:
On Wed, Sep 02, 2015 at 07:14:40PM -0400, Tom Smith wrote:
> Hi:
>
> I need to get the first and last tow in one sql like below
>
> select first(col1), last(col1) from table order by col1

Have you tried the window functions [1] last_value and first_value?

You could use something like:

SELECT first_value(col1) over (order by col1),
       last_value(col1)  over (order by col1)
FROM table;

It should be as simple as that, or I miss your point.


[1] http://www.postgresql.org/docs/current/static/functions-window.html


[]s
--
Dickson S. Guedes
@guediz - http://github.com/guedes

Re: get first and last row in one sql as two columns

From
Thomas Kellerer
Date:
Tom Smith schrieb am 03.09.2015 um 14:11:
>>
>>     SELECT first_value(col1) over (order by col1),
>>            last_value(col1)  over (order by col1)
>>     FROM table;
>>

> The window function works for me (with adding limit 1 in the end to output only one row
> needed instead of many duplicate rows).


If that works for you with a LIMIT 1, then I don't understand why

   select min(col1), max(col1)
   from table

doesn't work for you