What is the first name of the staff who did not give any lesson?
create table lessons (staff_id varchar, PRIMARY KEY (staff_id)); create table staff (first_name varchar, PRIMARY KEY (first_name)); create table staff (first_name varchar, staff_id varchar, PRIMARY KEY (first_name))
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