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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))

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

...
# might just rollback instead of deleting if the inserts
# were never commited.
DELETE FROM temp where id=?;