What are student ids who are allergic to milk?
select stuid from has_allergy where allergy = "Milk"
Of these, who also have allergy to cat?
select stuid from has_allergy where allergy = "Milk" intersect select stuid from has_allergy where allergy = "Cat"
List their last name and age.
select lname, age from student where stuid in (select stuid from has_allergy where allergy = "Milk" intersect select stuid from has_allergy where allergy = "Cat")