Which students are allergic to cats?
select stuid from has_allergy where allergy = "Cat"
Which students are not?
select stuid from student except select stuid from has_allergy where allergy = "Cat"
Of these, list the first name and sex of students who are allergic to milk.
select fname, sex from student where stuid in (select stuid from has_allergy where allergy = "Milk" except select stuid from has_allergy where allergy = "Cat")