Re: How to select avg(select max(something) from ...)

On 2/18/25 12:56, dfgpostgres wrote:
> Hi:
> psql 15.3
>
> I have a table with sets of observations, each set sharing an id.
> I want to get the average of the max of each set.
>
> id  |  val
> -----------
> 1     5.0
> 1     4.3
> 1     3.8
> 2     4.8
> 2     6.0
> 2     2.9
> 3     4.1
> 3     4.4
> 3     8.0
>
> So I want the avg of the max of the set where id=1 (5.0), where id=2
> (6.0), where id=3 (8.0) ~= 6.33...
>
> I tried this...
>
>               select
>                  avg(x.maxsz)
>                from
>                  dvm.dvm_events d,
>                  (select cast(max(size_g) as int) as maxsz
>                    from dvm.wa_du_profile_data
>                    where dvm_id=d.dvm_id) x
>                where
>                   d.project='foo' and
>                  <more conditions on d>
>
> It doesn't like that reference to "d.dvm_id) in that subquery.

create table wa_du_profile_data (id integer, val float);

insert into wa_du_profile_data values (1, 5.0),
(1, 4.3),
(1, 3.8),
(2, 4.8),
(2, 6.0),
(2, 2.9),
(3, 4.1),
(3, 4.4),
(3, 8.0);

with max_val as (select max(val) from wa_du_profile_data group by id)
select avg(max) from max_val;

6.333333333333333

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com