Re: MEDIAN as custom aggregate?

On Fri, 12 Oct 2001 12:38:12 -0700
"Josh Berkus" wrote:

> For those whose stats terminology is rusty, the "median" is the "middle"
> value in a distribution.  For example, if we had the following data:
> 
> Table ages
> person        age
> Jim        21
> Rusty        24
> Carol        37
> Bob        62
> Leah        78
> 
> Our Median would be Carol's age, 37.  This is a different figure from
> the Mean, or Average, which is 44.4.  Using the combination of the Mean
> and the Median you can do all kinds of interesting statistical analysis.
> 

In such case of this, there needs "nextval" in a query to deal with
a large number of rows. I think the following query, is not smart, 
will return the necessary rows (or an average of the rows). But even
so it may need considerable time...


-- (on 7.1.3)
create sequence seq_ages start 1;

select a1.age, a1.rank   -- or select avg(a1.age) from (select a0.person, a0.age, (nextval('seq_ages') - 1) as rank
   from (select *, setval('seq_ages',1)   -- to reset a sequence                 from ages                 order by age
 -- this insignificant "order by" is                                --  needed in order to work "setval"
)as a0          order by a0.age       ) as a1 where exists (select * from ages                  where a1.rank >=
(select(count(*)+1)/2 from ages)                    and a1.rank <= (select  count(*)/2+1  from ages)               )
 
;



Regards,
Masaru Sugawara