Обсуждение: last entry per person

I have a table which holds a user name and their results in exams. There
can be multiple entries per user. I am trying to return the last entry
for each user. I can get the last entry in the table using the order
by/limit method but how would this be applied per user. My table
definition is like the following:

> gradeid serial primary key,
> user text,
> grade char(1),
> entered timestamp,
Any help would be appreciated.
Regards
Garry

garry <garry@scholarpack.com> wrote:

> I have a table which holds a user name and their results in exams. There
> can be multiple entries per user. I am trying to return the last entry
> for each user. I can get the last entry in the table using the order
> by/limit method but how would this be applied per user. My table
> definition is like the following:
>
>> gradeid serial primary key,
>> user text,
>> grade char(1),
>> entered timestamp,
> Any help would be appreciated.
> Regards
> Garry

What about "select user, max(entered) from table group by user" ?

Btw.: user is a reserved word, don't use it as column-name.



Andreas
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On 03/02/2012 08:30, Chris Travers wrote:

On Fri, Feb 3, 2012 at 12:26 AM, garry <garry@scholarpack.com> wrote:

I have a table which holds a user name and their results in exams. There can be multiple entries per user. I am trying to return the last entry for each user. I can get the last entry in the table using the order by/limit method but how would this be applied per user. My table definition is like the following:

gradeid serial primary key,
user text,
grade char(1),
entered timestamp,

Any help would be appreciated.
Regards

What about a combination of a common table expression and a windowing function?  You ought to be able to order by your criteria and then pull where the rowcount in the window is 1.

Best Wishes,
Chris Travers

 

Garry



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There is no simple solution like there is for the last row in the table.
Window functions are one option, sub queries are another:

SELECT *
  FROM results,
           ( SELECT student_id,
                    max(result_date) result_date
               FROM results
           GROUP BY student_id
           ) as latest
 WHERE results.student_id = latest.student_id
   AND results.result_date = latest.result_date

Note that for this to work correctly, result_date will need to be unique.

Regards

On 03/02/2012 08:30, Chris Travers wrote:

On Fri, Feb 3, 2012 at 12:26 AM, garry <garry@scholarpack.com> wrote:

I have a table which holds a user name and their results in exams. There can be multiple entries per user. I am trying to return the last entry for each user. I can get the last entry in the table using the order by/limit method but how would this be applied per user. My table definition is like the following:

gradeid serial primary key,
user text,
grade char(1),
entered timestamp,

Any help would be appreciated.
Regards

What about a combination of a common table expression and a windowing function?  You ought to be able to order by your criteria and then pull where the rowcount in the window is 1.

Unfortunately I am using a version of postgres without the windowing functions, but that would have done the job.
Regards
Garry

On 2012-02-03, garry <garry@scholarpack.com> wrote:
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> I have a table which holds a user name and their results in exams. There
> can be multiple entries per user. I am trying to return the last entry
> for each user. I can get the last entry in the table using the order
> by/limit method but how would this be applied per user. My table
> definition is like the following:
>
>> gradeid serial primary key,
>> user text,
>> grade char(1),
>> entered timestamp,

select distinct on (user) * from EXAMS order by entered desc;

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