/* 3. */ SELECT c1, c2, ARRAY_TO_STRING(ARRAY_AGG((SELECT t3.c1 FROM s.t1 as t3 JOIN s.t1 as t2 ON t3.c3 = t2.c2)), ',') FROM s.t1 t1 GROUP BY c1; DROP SCHEMA s CASCADE;
The following adjustments should work:
array_to_string(select array_agg(t3.c1) FROM ... WHERE t2.c1 = t1.c1), ','
The array_agg needs to moved into the subquery - which causes an implicit GROUP BY to be added to the subselect thus ensuring only one row is returned for processing by the array_to_string function.
You need to add the where clause to make the subquery correlate to the outer query.
You may need to play with it a bit as I didn't try running your example.