combine multiple row values in to one row

Hi;

I'm looking for a way to do this:

# \d tbl
Table "public.tbl"
Column | Type | Modifiers
--------+---------+-----------
idn | integer |
code | text |
# SELECT * FROM tbl;
idn | code
-----+------
1 | A
2 | B
2 | C
3 | A
3 | C
3 | E
(6 rows)
# select idn, magic() as codes FROM tbl;
idn | codes
-----+------
1 | A
2 | B, C
3 | A, C, E
(3 rows)

Right now, I use plpgsql functions, but each time I do it
I have to rewrite the function to customize it.

Is there a generic way to do this? An aggregate maybe?

Thanks for any help.

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