Если еще актуально:

CREATE FUNCTION gc_init(dummy VARCHAR(255)) RETURNING LVARCHAR;
RETURN '';
END FUNCTION;

CREATE FUNCTION gc_iter(result LVARCHAR, value VARCHAR(255))
RETURNING LVARCHAR;
IF result = '' THEN
RETURN TRIM(value);
ELSE
RETURN result || ',' || TRIM(value);
END IF;
END FUNCTION;

CREATE FUNCTION gc_comb(partial1 LVARCHAR, partial2 LVARCHAR)
RETURNING LVARCHAR;
IF partial1 IS NULL OR partial1 = '' THEN
RETURN partial2;
ELIF partial2 IS NULL OR partial2 = '' THEN
RETURN partial1;
ELSE
RETURN partial1 || ',' || partial2;
END IF;
END FUNCTION;

CREATE FUNCTION gc_fini(final LVARCHAR) RETURNING LVARCHAR;
RETURN final;
END FUNCTION;

CREATE AGGREGATE group_concat
WITH (INIT = gc_init, ITER = gc_iter,
COMBINE = gc_comb, FINAL = gc_fini);