统计每个分类的人数

//直接按钮搜索
 private void button5_Click(object sender, EventArgs e)
 {
     string s = "select department.deptno as 系号,department.deptname as 系名,";
     s += "sum(class.num) as 人数 ";
     s += "from class,department where class.deptno=department.deptno ";
     s += "group by department.deptno,department.deptname";
     loadLV(listView2, s);
 }



//列表选择种类
        private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
        {
            string s = "select year as 年份,sum(num) as 学生人数 from class ";
            s += "where year =" + comboBox2.Text;
            s += " group by year";
            loadLV(listView2, s);
        }