Combining Two Tables

Okay, I've seen similar posts like this put up before and unfotunately
either they've never been answered or the answer didn't seem to help. So
here goes again :)

Basically I've got two tables. I've got a list of employees, each with a
unique number, and I've got a table of activities that track just what
they're up to.  It's conencted via a many-to-one relationship on that
employee number (emp_num).

So what I'm trying to do is get a list of all the employees who've done
a specific event. So the basic SQL would be

SELECT count(*), other_data, etc FROM employee, activity 
WHERE activity.emp_num=employee.emp_num AND other_criteria;

The problem with this is that it returns no records for those employees
who haven't done that that type of activity, eg. I'd get this:

emp_num | count | other
-----------------------
0000001       5     foo
0000002      11  foobar
0000004       6    foo2

instead of this, which is what I'd like to see:

emp_num | count | other
-----------------------
0000001       5     foo
0000002      11  foobar
0000003       0  numbar    <---- excluded record from first set
0000004       6    foo2

What I used to do was the following hack. I UNIONed the first SQL with
a second one, that looked like this:

SELECT 0, other_data, etc FROM employee, activity 
WHERE activity.emp_num=employee.emp_num AND other_criteria;

That worked. But I've just installed the  newest version of pgSQL and
that trick now returns two records for every employee, a zero and their
actual count. Strangely enough, even those employees who have zero
counts have two entries. Seeing that, I thought the original SQL with
just a straight count() would work now. It didn't. It still excludes
those employees with zero records (which makes sense). 

So my question is twofold:

i) Why am I getting two records for those employees with zero counts
when I use UNION now?

ii) More importantly, how do I get the result I want?

Any help would be *greatly* appreciated.

Wood Shavings! - Andrew