What are the students' first names who have both cats and dogs as pets?
create table has_pet (stuid varchar, petid varchar, PRIMARY KEY (stuid)); create table student (fname varchar, stuid varchar, PRIMARY KEY (fname)); create table pets (petid varchar, pettype varchar, PRIMARY KEY (petid))
select t1.fname from student as t1 join has_pet as t2 on t1.stuid = t2.stuid join pets as t3 on t3.petid = t2.petid where t3.pettype = 'cat' intersect select t1.fname from student as t1 join has_pet as t2 on t1.stuid = t2.stuid join pets as t3 on t3.petid = t2.petid where t3.pettype = 'dog'