Aggregate multiple string values to a single value, optionally delimited.
select d.department_id,
d.department_name,
string_agg(e.first_name, ',' order by first_name) as all_employees
from departments d
join employees e on d.department_id = e.department_id
group by d.department_id
order by d.department_id;
might return something like this:
department_id | department_name | all_employees
--------------+------------------+-------------------------
10 | Road consruction | Prostetnic Vogon Jeltz
20 | Hiking | Arthur, Ford
30 | Shipping | Zaphod
40 | Engineering | Marvin, Tricia