用vb实现数据库的隔离级别

 
比如下面这个例子，在更新考试成绩的时候，想设置READ COMMITTED应该怎么写
Sub changeExamResult()
        Dim str As String
        'Dim frmName As String = "欢迎您"
        str = "Data Source=localhost;Initial Catalog = Student;integrated Security=true"
        Dim con As New SqlConnection(str)
        con.Open()


        Dim sql As String = "select * from result_Info where exam_No='" & TextBox1.Text.ToString().Trim() & "' "
        'and user_Des='" & sMenuText & "'
        Dim sql2 As String = "update result_Info set student_ID='" & TextBox2.Text.ToString().Trim() & "',student_Name='" & TextBox3.Text.ToString.Trim() & "',term='" & ComboBox1.Text.ToString.Trim() & "',class_No='" & TextBox5.Text.ToString.Trim() & "',course_Name='" & TextBox6.Text.ToString.Trim() & "',result='" & TextBox7.Text.ToString.Trim() & "' where exam_No='" & TextBox1.Text.ToString().Trim() & "' "
        'director='" & TextBox2.Text.ToStr