SELECT LEFT(COLUMN_NAME, LENGTH(COLUMN_NAME) - 1)
FROM (
    SELECT COLUMN_NAME + ', '
    FROM information_schema.columns
    WHERE  table_name IN ('Employee')
    FOR XML PATH ('')
  ) c (COLUMN_NAME)

$ ./install testdb
CREATE AGGREGATE
Created uda
Done
CREATE AGGREGATE
Created uda
Done

TESTDB.ADMIN(ADMIN)=> select * from test_gc order by col1, col2;
 COL1 | COL2
------+------
    1 | A
    1 | B
    1 | C
    2 | C
    2 | D
    2 | E
(6 rows)

TESTDB.ADMIN(ADMIN)=> select col1, group_concat(col2,',') from test_gc group by col1 order by col1;
 COL1 | GROUP_CONCAT
------+--------------
    1 | A,B,C
    2 | C,D,E
(2 rows)