--select * from profs where plast like '%-%';
--select room from rooms where maxseats >= 10;
--select distinct slast from grades where grade<1 order by slast
--select * from students;
--select * from students s1, students s2 where s1.house='Gryffindor'
--  and s2.house='Slytherin'
--select * from students s1 JOIN students s2 ON s1.house='Gryffindor'
--  and s2.house='Slytherin'
--select students.slast, sfirst, crn, grade from grades join students 
--  on grades.slast=students.slast;
--select * from students join grades on slast
--select * from grades natural join students natural join courses
--select * from courses c1 join courses c2 on
--  c1.room = c2.room and c1.starttime = c2.starttime and c1.year = c2.year
--  and c1.name != c2.name and c1.crn < c2.crn
--select slast,crn,name,plast from heads natural join courses natural join students 
--  natural join grades
--select * from grades natural join students natural join courses
---  join heads on students.house = heads.house and 
-- courses.plast <> heads.plast
select * from students s1 natural join grades g1 
    join students s2 natural join grades g2
	on g1.grade > 2 and g2.grade <= 1 and g1.crn=g2.crn