SELECT events.b_g FROM events WHERE
 events.b_g NOT IN ((SELECT otf.b_g FROM otf) UNION
                    (SELECT requests.b_g FROM requests))

##</code><code>##

# insert all of the values you want to test with IN
INSERT INTO temp (id, value) VALUES (?, ?);

# now replace the placeholder IN with a sub-select
SELECT ...
 WHERE column IN (SELECT value FROM temp WHERE id=?);

# after you're completely done, remove the values -- you
# might just rollback instead of deleting if the inserts
# were never commited.
DELETE FROM temp where id=?;