Обсуждение: GROUP BY, ORDER & LIMIT ?

select class, size from X t1 where size in (select size from X t2
where t2.class=t1.class order by size desc limit 5);

On Tue, May 6, 2008 at 1:22 PM, Kynn Jones <kynnjo@gmail.com> wrote:

> Suppose table X has two columns: class (TEXT) and size (INT).  I want a
> listing showing the (up to) 5 largest values of "size" for each value of
> "class" (for some values of "class" the total number of available records
> may be less than 5).
>
> What would be the simplest way to achieve such a listing?  It seems like
> something one could do with GROUP BY, ORDER, and LIMIT, but I can't figure
> out the right syntax for this query.

Warning, this is typed directly into mail:
select class, size from X t1 where size in (select size from X t2
where t2.class=t1.class order by size desc limit 5);

That should do the trick. The only problem is if you've got duplicated
size values, you could end up with more than 5 per class.

--
- David T. Wilson
david.t.wilson@gmail.com

On Tue, May 06, 2008 at 01:22:30PM -0400, Kynn Jones wrote:
> Suppose table X has two columns: class (TEXT) and size (INT).  I want a
> listing showing the (up to) 5 largest values of "size" for each value of
> "class" (for some values of "class" the total number of available records
> may be less than 5).
> What would be the simplest way to achieve such a listing?  It seems like
> something one could do with GROUP BY, ORDER, and LIMIT, but I can't figure
> out the right syntax for this query.

get  rownum(in_code TEXT) function from
http://www.depesz.com/index.php/2007/08/17/rownum-anyone-cumulative-sum-in-one-query/
(it's important to use the version with in_code argument.

then write:

select *
from
(select class, size from X order by class asc size desc) q
where rownum(class) <= 5;

depesz

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