Postgres 11

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CREATE TABLE w (id bigint, clst int);

CREATE TABLE


INSERT INTO w (id,clst)
VALUES 
  (1,0),
  (1,4),
  (2,1),
  (2,2),
  (2,3),
  (3,2),
  (4,2),
  (5,4),
  (6,5);

INSERT 0 9


with recursive a as (
  select id, array_agg(distinct clst) clst from w group by id)
, t(id,pid,clst) as (
  select id,id,clst from a
  union all
  select t.id,a.id,t.clst|a.clst
  from t join a on a.id<>t.pid and t.clst&&a.clst and not t.clst@>a.clst)
, d as (
  select distinct on(id) id, clst from t order by id, cardinality(clst) desc)
select array_agg(id), clst from d group by clst;


array_agg
clst


{6}
{5}

{2,3,4}
{1,2,3}

{1,5}
{0,4}



SELECT 3