Postgres 13

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create table table_name (
  Organisation_id int,
  User_name varchar(10))
insert into table_name values
(1,'abc'),
(1,'abc'),
(1,'abc'),
(1,'abc'),
(1,'abc'),
(1,'abc'),
(1,'def'),
(1,'ghi'),
(1,'jkl'),
(1,'mno'),
(1,'pqr'),
(1,'stu'),
(1,'vwx'),
(1,'rst'),
(1,'abk'),
(1,'ruk'),
(2,'pqr'),
(2,'pqr'),
(2,'stu'),
(2,'vwx'),
(2,'rst'),
(2,'abk'),
(2,'ruk');
23 rows affected

With topTen as
(Select
  Organisation_id,
  User_name,
  Rank() over (
    partition by organisation_id 
    order by user_name) rn
From table_name 
group by 
  Organisation_id,
  user_name)
Select
  Organisation_id,
  String_agg(user_name,',') users
From topTen
Where rn <= 10
group by Organisation_id;


organisation_id
users


1
abc,abk,def,ghi,jkl,mno,pqr,rst,ruk,stu

2
abk,pqr,rst,ruk,stu,vwx

organisation_id	users
1	abc,abk,def,ghi,jkl,mno,pqr,rst,ruk,stu
2	abk,pqr,rst,ruk,stu,vwx