Select common values when using group by [Postgres] - postgresql

Question

I have three main tables meetings, persons, hobbies with two relational tables.
Table meetings
+---------------+
| id | subject |
+----+----------+
| 1 | Kickoff |
| 2 | Relaunch |
| 3 | Party |
+----+----------+
Table persons
+------------+
| id | name |
+----+-------+
| 1 | John |
| 2 | Anna |
| 3 | Linda |
+----+-------+
Table hobbies
+---------------+
| id | name |
+----+----------+
| 1 | Soccer |
| 2 | Tennis |
| 3 | Swimming |
+----+----------+
Relation Table meeting_person
+-----------------+-----------+
| id | meeting_id | person_id |
+----+------------+-----------+
| 1 | 1 | 1 |
| 2 | 1 | 2 |
| 3 | 1 | 3 |
| 4 | 2 | 1 |
| 5 | 2 | 2 |
| 6 | 3 | 1 |
+----+------------+-----------+
Relation Table person_hobby
+----------------+----------+
| id | person_id | hobby_id |
+----+-----------+----------+
| 1 | 1 | 1 |
| 2 | 1 | 2 |
| 3 | 1 | 3 |
| 4 | 2 | 1 |
| 5 | 2 | 2 |
| 6 | 3 | 1 |
+----+-----------+----------+
Now I want to to find the common hobbies of all person attending each meeting.
So the desired result would be:
+------------+-----------------+------------------------+
| meeting_id | persons | common_hobbies |
| | (Aggregated) | (Aggregated) |
+------------+-----------------+------------------------+
| 1 | John,Anna,Linda | Soccer |
| 2 | John,Anna | Soccer,Tennis |
| 3 | John | Soccer,Tennis,Swimming |
+------------+-----------------+------------------------+
My current work in progress is:
select
m.id as "meeting_id",
(
select string_agg(distinct p.name, ',')
from meeting_person mp
inner join persons p on mp.person_id = p.id
where m.id = mp.meeting_id
) as "persons",
string_agg(distinct h2.name , ',') as "common_hobbies"
from meetings m
inner join meeting_person mp2 on m.id = mp2.meeting_id
inner join persons p2 on mp2.person_id = p2.id
inner join person_hobby ph2 on p2.id = ph2.person_id
inner join hobbies h2 on ph2.hobby_id = h2.id
group by m.id
But this query lists not the common_hobbies but all hobbies which are at least once mentioned.
+------------+-----------------+------------------------+
| meeting_id | persons | common_hobbies |
+------------+-----------------+------------------------+
| 1 | John,Anna,Linda | Soccer,Tennis,Swimming |
| 2 | John,Anna | Soccer,Tennis,Swimming |
| 3 | John | Soccer,Tennis,Swimming |
+------------+-----------------+------------------------+
Does anyone have any hints for me, on how I could solve this problem?
Cheers

Answer 1

This problem can be solved by implement custom aggregation function (found it here):
create or replace function array_intersect(anyarray, anyarray)
returns anyarray language sql
as $$
select
case
when $1 is null then $2
when $2 is null then $1
else
array(
select unnest($1)
intersect
select unnest($2))
end;
$$;
create aggregate array_intersect_agg (anyarray)
(
sfunc = array_intersect,
stype = anyarray
);
So, the solution can be next:
select
meeting_id,
array_agg(ph.name) persons,
array_intersect_agg(hobby) common_hobbies
from meeting_person mp
join (
select p.id, p.name, array_agg(h.name) hobby
from person_hobby ph
join persons p on ph.person_id = p.id
join hobbies h on h.id = ph.hobby_id
group by p.id, p.name
) ph on ph.id = mp.person_id
group by meeting_id;
Look the example fiddle
Result:
meeting_id | persons | common_hobbies
-----------+-----------------------+--------------------------
1 | {John,Anna,Linda} | {Soccer}
3 | {John} | {Soccer,Tennis,Swimming}
2 | {John,Anna} | {Soccer,Tennis}