--select * from courses c1, courses c2
--where c1.room = c2.room and c1.starttime = c2.starttime
--      and c1.year = c2.year
--	  and c1.crn < c2.crn

--select * from grades natural join students
--         natural join courses join  heads
--		 on heads.house=students.house
--		 where courses.plast != heads.plast

--select slast from grades where crn=1
--except
--select slast from grades where crn=2

--select distinct plast from profs (vs courses)
--except 
--select distinct plast from courses where starttime=9

--select * from courses where starttime!=9

--select * from courses natural join rooms
--where maxseats >= 10

--select c1.year,c2.year,c1.plast,c2.plast,c1.name,c2.name from courses c1, courses c2 
--  where c1.year != c2.year
--  and c1.plast = c2.plast

-- Want to find the name of the head of Padma Patil's house?

--select * from students where slast='Patil'

--select plast from heads where house='Ravenclaw'

--select plast from heads where house=
--  (select house from students where slast='Patil')

select * from students natural join heads
where slast='Patil'