--(select slast from students) INTERSECT (select plast from profs)

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

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

--select name, year, plast from courses natural join rooms where maxseats >= 10

--select plast from courses except
--select c1.plast from courses c1 join courses c2 ON
--   c1.plast = c2.plast and c1.name != c2.name

-- NESTED SUBQUERIES

--select house from students where slast='Patil'
--select * from heads where house='Ravenclaw'

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

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

--select room from courses where crn=42
--select * from courses where room=(select room from courses where crn=42)

--select * from courses natural join heads

--select plast from heads;
--select * from courses where plast NOT IN (select plast from heads)

--select starttime from grades natural join courses where slast='Potter'
--   and year=1996

--select * from courses where year=1996 and starttime NOT IN
--  (select starttime from grades natural join courses where slast='Potter'
--   and year=1996)

--select year from courses where plast='Hagrid' and name like 'Care%'

select * from courses where name like 'Care%' and 
  year < all (select year from courses where plast='Hagrid' and name like 'Care%')