There are a few things to note here. I'm really not fond of "natural joins" as they simply disguise useful detail in my view, so I have not used them. I had to assume that the table "GROUP" joins via CONCENTRATOR_GROUP for an example of that missing detail.
The table name "GROUP" isn't a great idea as it is a very commonly used reserved word. I'd not recommend using such a word as a table name. Due to this "GROUP" is quoted (it isn't normal to quote object names in Oracle my experience).
You talk about "distinct" as if it has some magical quality that I should intuitively understand. It doesn't, and I don't. Let's say there are just 2 departments both are also "distinct"
DeptX
DeptY
So now let's assume there are 2 concentrators, both of these are "distinct" too:
ConcenA
ConcenB
Both concentrators are used in both departments, so we produce this query:
select distinct
c.name as c_name, d.name as d_name
from concentrators c
inner join departments d on c.dept_id=d.dept_id
The result is:
ConcenA DeptX
ConcenB DeptX
ConcenA DeptY
ConcenB DeptY
All 4 rows are "distinct"
The point is that "select distinct" is a "row operator", i.e. it considers the entire row to determine if any part of the row is different to all other rows. There are no subtleties or options to "select distinct", it always works the same way (over the entire row). So, with this in mind, we now know that "select distinct" simply is not going to be the right technique (and due to the technical definition of distinct you might also sense it isn't a good way to describe your problem either).
So, as "select distinct" isn't the right technique typically one can turn to these as techniques: "group by" or "row_number()"
because these do give us subtleties and options.
Now you haven't explained why or how you would choose just one department (in fact, to me, it sounds weird you would choose just one) but below I offer you A way to do this using row_number() and the "subtlety" being used is the ORDER BY which gives the number 1 to the first Department Name in alphabetic order, all other departments get more than 1; and this occurs for each CONCENTRATOR_ID because row_number() is "partitioned by" that field.
SELECT
department_name
, type_name
, NAME
, CONCENTRATOR_ID
, INTERNALADDRESS
, TYPE_ID
, DEPARTMENT_ID
FROM (
SELECT
d.NAME AS department_name
, t.NAME AS type_name
, c.CONCENTRATOR_ID
, c.NAME
, c.INTERNALADDRESS
, c.TYPE_ID
, c.DEPARTMENT_ID
, ROW_NUMBER() OVER (PARTITION BY c.CONCENTRATOR_ID
ORDER BY d.NAME, t.NAME, c.NAME) AS RN
FROM CONCENTRATOR c
LEFT OUTER JOIN CONCENTRATOR_GROUP cg
ON c.CONCENTRATOR_ID = cg.CONCENTRATOR_ID
LEFT OUTER JOIN "GROUP" g
ON cg.GROUP_ID = g.GROUP_ID
LEFT OUTER JOIN TYPE t
ON c.TYPE_ID = t.TYPE_ID
LEFT OUTER JOIN DEPARTMENT d
ON c.DEPARTMENT_ID = c.DEPARTMENT_ID
) sq
WHERE RN = 1 /* HERE is where we restrict output to one department per concentrator */
ORDER BY
NAME ASC
, CONCENTRATOR_ID
;
I have no reason to change the type of joins as you can see they remain as left outer joins - but I suspect there may be no valid reason for all or some of these. Do use the more efficient INNER JOIN if you can.