Help with query when 1 field is empty

[freddienet]

I have a DB with 4 related tables. The 2 tables in question are bookings and services. The bookings table includes booking #, customer numbers, date booked and quantity. The services table describes the services. What I would like to do is query the 2 tables to see which service has not been booked. Basically eliminate from the query the services that have been booked and display only the services that have no purchases. I have investigated using null values in the bookings table. Also "" as a string. Not to familiar with these yet.

Thanks
Freddie