Which last names are both used by customers and staff?
select last_name from customers intersect select last_name from staff
What is the most common first name of staff?
select first_name from staff group by first_name order by count ( * ) desc limit 1
What's the most common last name of staff?
select last_name from staff group by first_name order by count ( * ) desc limit 1
What are the first names of the customers with the last name Mertz?
select first_name from staff where last_name = 'Mertz'
What about the staff?
select first_name from staff where last_name = 'Mertz'