Question

I’m trying a new approach for a project that I’m working on and I’m just starting to learn about Access Databases. I using VB.net and my question is: How do you see if a record exists in the table of the database. I thought I had it understood but that is not the case. I’m creating a login and I want it to check if the Username that they typed in exists before it tries to compare what you typed with what’s in the database. I see alot of questions on how to do this…but not for VB.net and MS Access

Here’s my code:

Imports System.Data.OleDb
Public Class LoginForm1
    Dim provider As String
    Dim dataFile As String
    Dim connString As String
    Public myConnection As OleDbConnection = New OleDbConnection
    Public dr As OleDbDataReader
    Dim Errors As String
    Public Sub AccessAccountDatabase()
        provider = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source ="
        dataFile = "C:\Users\Richard\Documents\Visual Studio 2010\Projects\CybSol Journal Database\CybSol Journal Database\cgi-bin\Data.mdb"
        connString = provider & dataFile
        myConnection.ConnectionString = connString
        Errors = ""
        Try
            myConnection.Open()
            Dim str As String
            str = "SELECT * FROM Accounts WHERE Username='" & UsernameTxt.Text & "' AND Password='" & PasswordTxt.Text & "'"
            Dim cmd As OleDbCommand = New OleDbCommand(str, myConnection)
            dr = cmd.ExecuteReader
            dr.Read()

            If UsernameTxt.Text = dr("Username").ToString AndAlso PasswordTxt.Text = dr("Password").ToString Then
                Dim Welcome As String = "SELECT * FROM Accounts WHERE Real_Name=" & "Username"
                MsgBox("Welcome back " & dr("Real_Name") & "!")
            Else
                MsgBox("Login Failure")
            End If
            myConnection.Close()
        Catch ex As Exception
            MsgBox(ex.ToString)
        End Try
    End Sub

    Private Sub OkayBtn_Click(sender As System.Object, e As System.EventArgs) Handles OkayBtn.Click
        AccessAccountDatabase()
    End Sub
End Class

So now my question is… How do you get it to check if a record exists in the database, because when you type in the correct information (The correct username and password that exists in the database) it says welcome and all. But when you type in the wrong Username and/or Password it doesn’t work. Without the “Try Catch” statement the program just freezes. With the try catch it states this:

System.InvalidOperationException: No data exists for the row/column.
   at System.Data.OleDb.OleDbDataReader.DoValueCheck(Int32 ordinal)
   at System.Data.OleDb.OleDbDataReader.GetValue(Int32 ordinal)
   at System.Data.OleDb.OleDbDataReader.get_Item(String name)
   at CybSol_Journal_Database.LoginForm1.AccessAccountDatabase() in c:\users\richard\documents\visual studio 2010\Projects\CybSol Journal Database\CybSol Journal Database\LoginForm1.vb:line 36

Addition information: line 36 is this: If UsernameTxt.Text = dr("Username").ToString AndAlso PasswordTxt.Text = dr("Password").ToString Then

Answer 1

First problem:

PASSWORD is a reserved keyword in Access. You should encapsulate in square brackets:

"SELECT * FROM Accounts WHERE Username='" & UsernameTxt.Text & _
"' AND [Password]='" & PasswordTxt.Text & "'" 

Second problem:

NEVER use string concatenation to create sql text. ALWAYS use parameters

 str = "SELECT * FROM Accounts WHERE Username=? AND [Password]=?"   
 Dim cmd As OleDbCommand = New OleDbCommand(str, myConnection)   
 cmd.Parameters.AddWithValue("user", UserNameTxt.Text)
 cmd.Parameters.AddWithValue("pass", PasswordTxt.Text)
 dr = cmd.ExecuteReader   

Why? look here what could happen if you concatenate strings from user input

Third problem: Test if your command returns rows

 If dr.Read() Then
    ......

 End if