SQL-запрос для предметного предварительного условия

Комментарии в очереди.

Private conString As String = "Your connection string"

Private Sub EnrollStudent(StudentID As Integer, SubjectID As Integer)
    'Question Does Subject have a prerequisite?
    Dim Prerequisite As Integer
    Using cn As New MySqlConnection(conString)
        Using cmd As New MySqlCommand("Select pre_id from subject_bsit 
                                        where subject_id = @subject_id", cn)
            cmd.Parameters.Add("@subject_id", MySqlDbType.Int32).Value = SubjectID
            cn.Open()
            Prerequisite = CInt(cmd.ExecuteScalar)
        End Using
    End Using
    'Answer - No, It is OK to enroll student there are no prerequisites
    If Prerequisite = 0 Then
        InsertEnrollment(StudentID, SubjectID)
        Return
    End If
    'Answer - Yes there Is a prerequisite
    'New Question - Has the student passed the prerequisite?
    Dim PassingGrade As Integer
    Using cn As New MySqlConnection(conString)
        Using cmd As New MySqlCommand("Select Grade From student_subject 
                                        Where subject_id = @subject_id 
                                        And student_id = @student_id", cn)
            cmd.Parameters.Add("@subject_id", MySqlDbType.Int32).Value = Prerequisite
            cmd.Parameters.Add("@student_id", MySqlDbType.Int32).Value = StudentID
            cn.Open()
            PassingGrade = CInt(cmd.ExecuteScalar)
        End Using
    End Using
    'Answer - Yes, student has a passing grade.
    If PassingGrade > 0 Then
        InsertEnrollment(StudentID, StudentID)
    Else 'Answer - No, student does not have a passing grade in prerequisite.
        MessageBox.Show("Student cannot enroll because of prerequisite.")
    End If
End Sub

Private Sub InsertEnrollment(StudentID As Integer, SubjectID As Integer)
    Dim query = "Insert Into student_subject (student_id, subjectId, Grade, Enrolled) 
                Values (@student_id, @subject_id, 0, 1);"
    Using cn As New MySqlConnection(conString)
        Using cmd As New MySqlCommand(query, cn)
            cmd.Parameters.Add("@student_id", MySqlDbType.Int32).Value = StudentID
            cmd.Parameters.Add("@subject_id", MySqlDbType.Int32).Value = SubjectID
            cn.Open()
            cmd.ExecuteNonQuery()
        End Using
    End Using
    MessageBox.Show("Successful enrollment")
End Sub

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    EnrollStudent(1235, 5)
End Sub

РЕДАКТИРОВАТЬ

Опечатка здесь...

If PassingGrade > 0 Then
    InsertEnrollment(StudentID, StudentID)

Это должно быть

If PassingGrade > 0 Then
    InsertEnrollment(StudentID, SubjectID)

Второй аргумент — SubjectID.