query to find references

Hi you SQL guru's out there, please help me with the following query:

I have a table A and a table B. Rows in B contain a foreign key to A. I want to find all instances of A to which less than a certain number, say N, rows in B refer. A naive query would look like this but is not accepted by Oracle7 ('a' is the name of the key field):

SELECT * FROM A
WHERE (SELECT COUNT(*) FROM B
       WHERE B.a = A.a) < N

Why doesn't this simple query work?
How should I get what I want?

Thanks for any help,
Michiel

-- 
Michiel Perdeck


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