单条件，组合条件查询

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            string s = "select department.deptname as 系名称, class1.specname as 专业名称,";
            s += "student.sname as 学生姓名 from student,class1,department ";
            s += "where student.classno=class1.classno and class1.deptno=department.deptno ";
            if (comboBox1.Text.Trim().Length != 0 && comboBox1.Text != "请选择")
            {
                s += "and student.sname='" + comboBox1.Text + "'";
            }
            if (comboBox2.Text.Trim().Length != 0 && comboBox2.Text != "请选择")
            {
                s += "and department.deptname='" + comboBox2.Text + "'";
            }
            if (comboBox3.Text.Trim().Length != 0 && comboBox3.Text != "请选择")
            {
                s += "and class1.specname='" + comboBox3.Text + "'";
            }
            s += "order by class1.specname desc";
            loadLV(listView1, s);
        }







            this.comboBox1.Items.Insert(0, "请选择");
            this.comboBox2.Items.Insert(0, "请选择");
            this.comboBox3.Items.Insert(0, "请选择");