C#学生成绩查询系统实现

 private void 成绩查询_Load(object sender, EventArgs e)
        {
            sqlconnection1 = new SqlConnection(conn);
            str = "select course.course_name,student_course.student_id,student_course.score,student_course.credit,student_course.sc_semester,student_course.school_year from student_course inner join course on student_course.course_id=course.course_id";
            sqldataadapter1 = new SqlDataAdapter(str, sqlconnection1);
            dataset1 = new DataSet();
            dataset1.Clear();
            sqldataadapter1.Fill(dataset1, "course");
            dataGridView1.DataSource = dataset1.Tables[0];
        }

        private void btnquerybyname_Click(object sender, EventArgs e)
        {
            sqlconnection1 = new SqlConnection(conn);
            str = str = "select course.course_name,student_course.student_id,student_course.score,student_course.credit,student_course.sc_semester,student_course.school_year from student_course inner join course on student_course.course_id=course.course_id";
            str = str + " and course.course_name like '%" + txtstuname.Text.Trim() + "%' ";
            sqldataadapter1 = new SqlDataAdapter(str, sqlconnection1);
            dataset1 = new DataSet();
            dataset1.Clear();
            sqldataadapter1.Fill(dataset1, "course");
            dataGridView1.DataSource = dataset1.Tables[0];
        }

        private void bndelected_Click(object sender, EventArgs e)
        {
            if (MessageBox.Show("您确认要删除该记录吗？", "确认", MessageBoxButtons.YesNoCancel) == DialogResult.Yes)
            {
                string stu_id = this.dataGridView1.CurrentRow.Cells[1].Value.ToString();//获取选中行的学生ID号

                str = "delete from student_course where student_id=" + stu_id + " ";