Query1 selects all the PNo from project table.
Query2 is a correlated query that returns that Pno in which an employee works.
Query1 except Query2. returns in which project that particular employee did not work.
if Query1 except Query2 is an empty set , the outer query returns the employee name.
So we can say, Query1 except Query2 is an empty set if an employee works in all project.