What are the ids of the students who have any allergy?
select stuid from has_allergy
Of these, who are allergy to milk or cat?
select stuid from has_allergy where allergy = "Milk" or allergy = "Cat"
What are the different first names and city codes of them?
select distinct t1.fname, t1.city_code from student as t1 join has_allergy as t2 on t1.stuid = t2.stuid where t2.allergy = "Milk" or t2.allergy = "Cat"