What are the names and genders of staff who have held the title Sales Person, but never Clerical Staff?
select t1.staff_name, t1.staff_gender from staff as t1 join staff_department_assignments as t2 on t1.staff_id = t2.staff_id where t2.job_title_code = "Sales Person" except select t1.staff_name, t1.staff_gender from staff as t1 join staff_department_assignments as t2 on t1.staff_id = t2.staff_id where t2.job_title_code = "Clerical Staff"