Making a table with max 1 row

[AmFreak]

Hi,
i need a table with 1 or 0 rows in it. So that after 1 row is inserted the  
table can only be updated or the row deleted, but there can never be a row  
inserted.
I came as far as this:

CREATE TABLE test
 (field1 INTEGER NOT NULL
 CHECK (field1 = 1) UNIQUE)

but that would (obviously) mean that field1 could only have the value 1.

I would need something like check(COUNT(*) FROM test <= 1).

Thanks for any suggestions / answers

Biene Maja