What is the average salary for the physics department?
select avg ( salary ) from instructor where dept_name = 'Physics'
How many instructors in the physics department make less than that?
select count ( * ) from instructor where dept_name = 'Physics' and salary < ( select avg ( salary ) from instructor where dept_name = 'Physics' )
What is the name and salary of that instructor?
select name, salary from instructor where dept_name = 'Physics' and salary < ( select avg ( salary ) from instructor where dept_name = 'Physics' )