统计只上一门课的老师

        private void button5_Click(object sender, EventArgs e)
        {
            string s = "select teachers.tname as 姓名,courses.cnum as 课程号,courses.cname as 课程名,";
            s += "courses.ctime as 课时 from teachers,courses ";
            s += "where courses.tnum=teachers.tnum ";
            s += "and teachers.tnum in (select tnum from courses group by tnum having count(cnum)=1)";
            loadLV(listView2, s);
        }