list the name of all scientists.
select name from scientists
which of them are assigned to any project? List their names
select name from scientists where ssn in (select scientist from assignedto)
how many of those scientists are there?
select count(*) from scientists where ssn in (select scientist from assignedto)
Find the names of those who are not assigned to a project yet.
select name from scientists where ssn not in (select scientist from assignedto)