How To Define This In One Query?

Hi, here am i back ..

not sure it's possible to solve it in one query ...

there are four tables:

1) headquarters
---------------
hqID (primary key)
hqname

2) department
---------------
depID (primary key)
depname
hqID

3) reports
----------
repID (p. key)
depID
userID

4) users
--------
userID (p.key)
username

I want to get in one query those four fields:
hqID, hqname, depID, depname + varchar(number of distinct users that has
made a report for
each department) . This last field is a concatenation.

The complexity resides in the fact there is need for a COUNT among other
fields ...


E.g.: table reports may look like this:
 repID    depID    userID
    1        1            1
    2        1            1
    3        1            3
    4        3            6
    5        4            8

This gives:
for dep 1: 2 distinct users
for dep 2: 0
for dep 3: 1 distinct user
for dep 4: 1 distinct user

Thanks for help
Tartuffe