private void cbSubject_SelectedIndexChanged(object sender, EventArgs e)
{
teacherName = cbSubject.SelectedValue.ToString();
dtTeacher = StaticSQLiteHelper.GetSubjectTeacher(teacherName);
cbTeacher.DisplayMember = "教师姓名";
cbTeacher.ValueMember = "教师编号";
cbTeacher.DataSource = dtTeacher;
cbTeacher.SelectedIndex = 0;
}