Re: last entry per person

On 03/02/2012 08:30, Chris Travers wrote:
>
>
> On Fri, Feb 3, 2012 at 12:26 AM, garry <garry(at)scholarpack(dot)com
> <mailto:garry(at)scholarpack(dot)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