question: What is the last name of every student who is either female or living in a city with the code BAL or male and under 20?
#1 return students 
#2 return #1 who  are  female 
#3 return cities of  #1 
#4 return codes of  #3 
#5 return #1 where #4 is  BAL 
#6 return #1 who  are  male 
#7 return #6 who  are  under 20 
#8 return #2 ,  #5 ,   #7