查询至少选修课程号为'c2'和'c4' 两门课程的学生学号

选课表sc = {学号sco,课程号cno...}

--查询至少选修课程号为“c2”和“c4”两门课程的学生的学号
select sca.sno from sc as sca join sc as scb on sca.sno=scb.sno 
where sca.cno='c2' and scb.cno='c4';