Forgive me if this is an easy question. I teach classes and want to be able to select those students who have taken one class, but not another class. I have two tables: lessons_slots which is the table for every class such as:

--------------------
-ID name slots-
-1 basics 10 -
-2 advanced 10 -
-3 basics 10 -
---------------------

The other table is class_roll, which holds enrollment info, such as:

--------------------
-sID classid firstname lastname-
-1 1 Jo Schmo
-2 1 Person Two
...
-13 2 Jo Schmo
---------------------

What I want to do, I select everyone who has not had the advanced class (for example). I've tried doing

SELECT *
FROM lessons_slots
LEFT JOIN class_roll
ON lessons_slots.ID = class_roll.classid
WHERE lessons_slots.name != 'advanced'

But that doesn't work. Any ideas?