SELECT COUNT查询不会产生我期待的结果

Question

我有一个SELECT COUNT查询，用于确定用户是否已获得成绩，如果该特定用户已有成绩，则提交成绩的按钮将变为不可见。但是，即使我已经给了该学生一个成绩，按钮仍然会出现。当我在调试模式下启动时，查询的值为null。以下是方法中的代码：

    String connectionString = WebConfigurationManager.ConnectionStrings["UniString"].ConnectionString;
    SqlConnection myConnection = new SqlConnection(connectionString);

    myConnection.Open();
    String modOnpwayModID = "SELECT id FROM module_on_pathway WHERE module_id = '" + modDropDown.SelectedValue + "'";
    SqlCommand modOnpwayModIDQuerycmd = new SqlCommand(modOnpwayModID, myConnection);
    Int32 modOnpwayModIDResult = Convert.ToInt32(modOnpwayModIDQuerycmd.ExecuteScalar().ToString());
    Label lb = (Label)e.Item.FindControl("user_idLabel");
    String userIDLabel = lb.Text.ToString();
    Int32 usrIDVal = Convert.ToInt32(userIDLabel);

    String gradeSelectionQuery = "SELECT COUNT(student_module_grade.grade) FROM student_module_grade INNER JOIN classlist ON student_module_grade.classlist_id = classlist.classlist_id INNER JOIN student_assignment_grade ON student_module_grade.classlist_id = student_assignment_grade.classlist_id INNER JOIN assignments ON student_assignment_grade.assignment_id = assignments.assignment_id WHERE student_module_grade.module_on_pathway_id ='" + modOnpwayModIDResult + "'AND classlist.user_id = '" + userIDLabel + "'";


    SqlCommand gradeSelectionQuerycmd = new SqlCommand(gradeSelectionQuery, myConnection);
    Int32 gradeCount = Convert.ToInt32(gradeSelectionQuerycmd.ExecuteScalar().ToString());

    //See if a final score has been given already- can then be changed by the admin if it needs to be changed
    if (gradeCount == 0)
    {

        submitmodMark1st.Visible = true;

        //All elements for grade submission made invisible- admin will be unable to change the grade
        //TB.Visible = false;
        //changedFlagVal.Text = "The grade for this module has already been changed for the selected student";
        //changedFlagVal.Visible = true;
        //changedFlagVal.ForeColor = System.Drawing.Color.Red;

    }

    else
    {

        submitmodMark1st.Visible = false;
    }

String repeatGradeSelectionQuery = "SELECT COUNT(student_module_repeat_grades.grade) FROM student_module_repeat_grades INNER JOIN classlist ON student_module_repeat_grades.classlist_id = classlist.classlist_id INNER JOIN student_assignment_grade ON student_module_repeat_grades.classlist_id = student_assignment_grade.classlist_id INNER JOIN assignments ON student_assignment_grade.assignment_id = assignments.assignment_id WHERE student_module_repeat_grades.module_on_pathway_id ='" + modOnpwayModIDResult + "'AND classlist.user_id = '" + userIDLabel + "'";
    SqlCommand repeatGradeSelectionQuerycmd = new SqlCommand(repeatGradeSelectionQuery, myConnection);
    Int32 repeatGradeCount = Convert.ToInt32(repeatGradeSelectionQuerycmd.ExecuteScalar().ToString());

    if (repeatGradeCount == 0)
    {

        submitmodMark1st.Visible = true;

            }

            else
            {

                //All elements for grade submission made invisible- admin will be unable to change the grade
                //TB.Visible = false;
                changedFlagVal.Text = "The grade for this module has already been changed for the selected student";
                changedFlagVal.Visible = true;
                changedFlagVal.ForeColor = System.Drawing.Color.Red;
                submitmodMark1st.Visible = false;
            }