假设有一张成绩表（score）：字段如下： sid：学生学号， cid：课程编号， score：成绩 求所修课程完全相同的同学的学号。这个问题其实关键在于求课程集合相等的问题。 可以使用如下sql即可求出两个所修课程完全相同的同学的学号：
select s1.sid,s2.sid from score s1 inner join score s2 on(s1.cid=s2.cid and s1.sid < s2.sid) group by s1.sid,s2.sid having count(*) = (select count(*) from score where s1.sid=sid) and count(*) = (select count(*) from score where s2.sid=sid);
集合相等的问题可以衍生出如求与学号002同学所修课程完全相同的其它同学的学号和姓名：
select st.sid,st.sname from student st where st.sid in ( select sc.sc_sid from ( select s1.sid,s2.sid as sc_sid from score s1 inner join score s2 on(s1.cid=s2.cid and s1.sid s2.sid and s1.sid='002') group by s1.sid,s2.sid having count(*) = (select count(*) from score where sid=s1.sid) and count(*) = (select count(*) from score where sid=s2.sid) ) sc );