Finding records in one table not present in another table you could use either of these two methods.
For example, you want to see all of the registered users who have yet to place an order.

SELECT users.*
FROM users
LEFT JOIN uc_orders ON users.uid = uc_orders.uid
WHERE uc_orders.uid IS NULL

Or if joins are not supported you could use the slower method


SELECT users.*
FROM users
WHERE users.uid NOT IN (SELECT uid FROM uc_orders)