HAVING - clause

I have a data base for storing information on radioactive contamination in
various samples. I want to dig out for a set of the samples the first year
for which I have information for the sample types and a radio nuclides and
the average activity for that year. The relevant part of my data base are
two tables, sample and measure, which a bit simplified can be described as

create table sample(
id serial primary key,
type char()
year integer
)

create table measure(
id serial primary key,
sample id integer references sample(id),
nuclide char(),
value float
)


I tried to do a 
select year as First_year,sample.type,avg(value),nuclide
from sample,measure
where sample.id = sampleid
group by sample.type,nuclide
having year=min(year)

But it told me that year has to be grouped to be used in this way, so I
grouped, but then I got averages for each year, and the same result if I
also put a min(year) in the select part of the statement...

If I do a 
select min(year) as First_year,sample.type,nuclide
from sample,measure
where sample.id = sampleid
group by sample.type,nuclide

I get the first year for each sample type and nuclide, but then I need the
average for each of those first years... I thought I should use having for
that, but I have obiously overlooked or misunderstood something..

Any clues?

Morten

-- 
Morten Sickel
Norwegian Radiation Protection Authority