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')