Database: pets_1
Question: What is the first name of every student who has a dog but does not have a cat?
SQL: select t1.fname ,  t1.age 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' and t1.stuid not in (select t1.stuid 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')