Only one Select clause is allowable.
Schema (* is the key):
 student(sid*, sname, sex, age, year, gpa)
 dept(dname*, numphds)
 prof(pname*, dname)
 course(cno*, cname, dname*)
 major(dname*, sid*)
 section(dname*, cno*, sectno*, pname)
 enroll(sid*, grade, dname*, cno*, sectno*)

Find all pairs of different students that are enrolled in two or more sections together. Show just the sids, and don't repeat pairs of sids in the output.