CREATE TABLE Staff (first_name VARCHAR)
CREATE TABLE Lessons (staff_id VARCHAR)

What is the first name of the staff who did not give any lesson?
SELECT first_name FROM Staff EXCEPT SELECT T2.first_name FROM Lessons AS T1 JOIN Staff AS T2 ON T1.staff_id = T2.staff_id
SELECT "first_name" FROM "Staff" EXCEPT SELECT "T2"."first_name" FROM "Lessons" AS "T1" JOIN "Staff" AS "T2" ON "T1"."staff_id" = "T2"."staff_id"