extend "group by" to include "empty relations" ?

I've two tables related via a id-field.

Table "public.fn_kat"
Column | Type |
-----------------+-----------------------------+-
id | integer |
kategorie | text |

Table "public.fn_dokumente"
Column | Type |
-----------------+-----------------------------+-
kategorie | integer |
content | text |

I now want to list all entries in table fn_kat and count the number of
entries in fn_dokumente that have the actual id.

# select k.kategorie,count(d.oid) from fn_kat k,fn_dokumente d where
k.id=d.kategorie group by k.kategorie;

kategorie | count
------------------------------------------+-------
1. Forschungsnetzwerk Erwachsenenbildung | 1
1.1. Protokolle | 3
2. Sonstige Dokumente | 1

But there is a problem now: There are also entries in fn_kat which dont
have corresponding entries in fn_dokumente and this entries should be
listed too. With the proper count=0 !!

How to achieve this?

thnx a lot,
peter

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