Postgres 11

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CREATE VIEW v (a,b) AS (
  VALUES ('11111111', '1A'),
         ('11111111', '2A'),
         ('22222222', '3A'),
         ('33333333', '4a'),
         ('33333333', '5A'),
         ('33333333', '6A'));
SELECT string_agg(a || b, ', ' ORDER BY a || b)
FROM   v
GROUP  BY a
HAVING COUNT( a ) > 1
ORDER  BY 1;


string_agg


111111111A, 111111112A

333333334a, 333333335A, 333333336A


SELECT string_agg(a || b, ', ' ORDER BY a || b)
FROM   v
HAVING COUNT( a ) > 1
ORDER  BY 1;


string_agg


111111111A, 111111112A, 222222223A, 333333334a, 333333335A, 333333336A