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