Grouping user id columns together with string_agg on PostgreSQL 13 - postgresql

Question

This is my emails table
create table emails (
id bigint not null primary key generated by default as identity,
name text not null
);
And contacts table:
create table contacts (
id bigint not null primary key generated by default as identity,
email_id bigint not null,
user_id bigint not null,
full_name text not null,
ordering int not null
);
As you can see I have user_id field here. There can be multiple same user ID's on my result so i want to join them using comma ,
Insert some data to the tables:
insert into emails (name)
values
('dennis1'),
('dennis2');
insert into contacts (id, email_id, user_id, full_name, ordering)
values
(5, 1, 1, 'dennis1', 9),
(6, 2, 1, 'dennis1', 5),
(7, 2, 1, 'dennis1', 1),
(8, 1, 3, 'john', 2),
(9, 2, 4, 'dennis7', 1),
(10, 2, 4, 'dennis7', 1);
My query is:
select em.name,
c.user_ids
from emails em
join (
select email_id, string_agg(user_id::text, ',' order by ordering desc) as user_ids
from contacts
group by email_id
) c on c.email_id = em.id
order by em.name;
Actual Result
name user_ids
dennis1 1,3
dennis2 1,1,4,4
Expected Result
name user_ids
dennis1 1,3
dennis2 1,4
On my real-world data, I get same user id like 50 times. Instead it should appear 1 time only. In example above, you see user 1 and 4 appears 2 times for dennis2 user.
How can I unique them?
Demo: https://dbfiddle.uk/?rdbms=postgres_13&fiddle=2e957b52eb46742f3ddea27ec36effb1
P.S: I tried to add user_id it to group by but this time I get duplicate rows...

Answer 1

demo:db<>fiddle
SELECT
name,
string_agg(user_id::text, ',' order by ordering desc)
FROM (
SELECT DISTINCT ON (em.id, c.user_id)
*
FROM emails em
JOIN contacts c ON c.email_id = em.id
) s
GROUP BY name
Join the tables
DISTINCT ON email and the user_id, so for every email record, there is no equal users
Aggregate