Thanks for helping Terry! Below is the code!

My database consist of a table called Credential with 4 fields Username, Password, LoginFlag and SessionID

SELECT STATEMENT seems fine but UPDATE is NOT working.

 I am pretty sure my code is correct. I follow the code from asp.net workbook by Heng Ngee Mok.

Imports System.Data.OleDb
Public Class Authentication
    Inherits System.Web.UI.Page


    Dim cnn As New OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=c:\inetpub\wwwroot\MMORPGapi\MMORPG.mdb")

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Dim asd As String = HttpContext.Current.Request("username")
        testLabel.Text = asd

        Dim username As String = "jianda"
        Dim pwd As String = "jiandapass"
        Dim SID As String = "tester"
        UsernameAndPwdAreValid(username, pwd, SID)

        Response.Write("Hello JD!")
    End Sub


    Private Sub UsernameAndPwdAreValid(ByVal username As String, ByVal pwd As String, ByVal SID As String)
        Dim sqlStr As String = "SELECT COUNT(*) AS NUMBEROFROWS FROM Credential WHERE [Username] = '" + username + "' AND [Password] = '" + pwd + "'"

        Dim cmd As New OleDbCommand(sqlStr, cnn)
        Dim dataReader As OleDbDataReader
        Trace.Write(sqlStr)



        Try
            cnn.Open()
            dataReader = cmd.ExecuteReader()

            Dim counter As Integer

            Do While dataReader.Read
                counter = Convert.ToInt16(dataReader("NUMBEROFROWS"))
            Loop

            If counter = 1 Then
                dataReader.Close()
                Dim online As String = "online"
                sqlStr = "UPDATE Credential SET [LoginFlag]='" + online + "', [SessionID]='" + SID + "' WHERE [Username]='" + username + "'"
                Trace.Write(sqlStr)
                cmd.ExecuteNonQuery()
            End If


        Catch ex As Exception

        Finally
            If Not cnn Is Nothing Then
                cnn.Close()
                cnn.Dispose()
            End If
        End Try


    End Sub

    
End Class