Using value in 1 table to count records in another

Heyas;

I am a bit stuck with a task at work.

There are two tables: CLIENTS & DOCUMENTS.

CLIENTS TABLE(ClientID, Status)
DOCUMENTS TABLE(id, clientid, created, createdby)

Here's the steps I need:

1. I need to count the number of DOCUMENTS per ClientID.
2. IF COUNT = 0

UPDATE CLIENTS
SET STATUS = 'BYPASS'

That's it...but I am stuck on it. I have tried everything I can think of, cursors, a simple UPDATE command...and nothing. I think I am just looking at this problem wrong. I was hoping someone on here would be willing to show me what I'm doing wrong.

I have even tried something like:

IF (SELECT COUNT(*) FROM DOCUMENTS WHERE ClientID = DOCUMENTS.ClientID) = 0
BEGIN
UPDATE CLIENTS
SET STATUS = 'BYPASS'
END

it didn't seem to work....can someone PLEASE help me!